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UAI 2015 - Accepted Papers

ID: 9 (pdf)
Bethe and Related Pairwise Entropy Approximations +
For undirected graphical models, belief propagation often performs remarkably well for approximate marginal inference, and may be viewed as a heuristic to minimize the Bethe free energy. Focusing on binary pairwise models, we demonstrate that several recent results on the Bethe approximation may be generalized to a broad family of related pairwise free energy approximations with arbitrary counting numbers. We explore comparisons to the true (Gibbs) free energy and shed light on the empirical success of the Bethe approximation.
Adrian Weller, University of Cambridge
ID: 12 (pdf)
Tracking with ranked signals +
We present a novel graphical model approach for a problem not previously considered in the machine learning literature: that of tracking with ranked signals. The problem consists of tracking a single target given observations about the target that consist of ranked continuous signals, from unlabeled sources in a cluttered environment. We introduce appropriate factors to handle the imposed ordering assumption, and also incorporate various systematic errors that can arise in this problem, particularly clutter or noise signals as well as missing signals. We show that inference in the obtained graphical model can be simplified by adding bipartite structures with appropriate factors. We apply a hybrid approach consisting of belief propagation and particle filtering in this mixed graphical model for inference and validate the approach on simulated data. We were motivated to formalize and study this problem by a key task in Oceanography, that of tracking the motion of RAFOS ocean floats, using range measurements sent from a set of fixed beacons, but where the identities of the beacons corresponding to the measurements are not known. However, unlike the usual tracking problem in artificial intelligence, there is an implicit ranking assumption among signal arrival times. Our experiments show that the proposed graphical model approach allows us to effectively leverage the problem constraints and improve tracking accuracy over baseline tracking methods yielding results similar to the ground truth hand-labeled data.
Tianyang Li, UT Austin; Harsh Pareek, UT Austin; Pradeep Ravikumar, UT Austin; Dhruv Balwada, Geophysical Fluid Dynamics Institute at Florida State University; Kevin Speer, Geophysical Fluid Dynamics Institute at Florida State University
ID: 13 (pdf)
The Long-Run Behavior of Continuous Time Bayesian Networks +
The continuous time Bayesian network (CTBN) is a temporal model consisting of interdependent continuous time Markov chains (Markov processes). One common analysis performed on Markov processes is determining their long-run behavior, such as their stationary distributions. While the CTBN can be transformed into a single Markov process of all nodes' state combinations, the size is exponential in the number of nodes, making traditional long-run analysis intractable. To address this, we show how to perform "long-run" node marginalization that removes a node's conditional dependence while preserving its long-run behavior. This allows long-run analysis of CTBNs to be performed in a top-down process without dealing with the entire network all at once.
Liessman Sturlaugson, Montana State University; John Sheppard, Montana State University
ID: 16 (pdf)
Complexity of the Exact Solution to the Test Sequencing Problem +
Consider a doctor choosing a treatment for an uncertain disorder for which there are n costly tests available. Based on the test results observed so far, the doctor can either order another test or proceed to the treatment decision. Although test sequencing is a problem that arises frequently in many decision situations, finding an exact solution is NP-hard with respect to n. In this paper, we analyze the time complexity of classic symmetric and asymmetric formulations, using influence diagrams and decision trees, to the general test sequencing problem, making no assumptions of conditional independence among the test results. We develop an alternative influence diagram formulation that scales better, and show how a decision circuit formulation improves on the decision tree solution through recursive coalescence. We prove that this decision circuit formulation achieves the lower bound complexity for any method for the general test sequencing problem that examines the entire policy space. As a result, the problem is tractable for much larger n than has been possible to date.
Wenhao Liu, Stanford University; Ross Shachter, Stanford University
ID: 31 (pdf)
Budget Constraints in Prediction Markets +
An automated market maker is a natural and common mechanism to subsidize information acquisition, revelation, and aggregation in a prediction market. The sought-after prediction aggregate is the equilibrium price. However, traders with budget constraints are restricted in their ability to impact the market price on their own. We give a detailed characterization of optimal trades in the presence of budget constraints in a prediction market with a cost-function-based automated market maker. As a concrete application of our characterization, we give sufficient conditions for a property we call budget additivity: two traders with budgets B and B' and the same beliefs would have a combined impact equal to a single trader with budget B+B'. That way,even if a single trader cannot move the market much, a crowd of like-minded traders can have the same desired effect. We show that a generalization of the heavily-used logarithmic market scoring rule is budget additive for affinely independent pay- offs, but the quadratic market scoring rule is not. Our results may be used both descriptively, to understand if a particular market maker is affected by budget constraints or not, and prescriptively, as a recipe to construct markets.
Nikhil Devanur, Microsoft Research; Miroslav Dudik, Microsoft Research; Zhiyi Huang, University of Hong Kong; David Pennock, Microsoft Research
ID: 33 (pdf)
Adversarial Cost-Sensitive Classification +
In many classification settings, mistakes incur different application-dependent penalties based on the predicted and the actual class label. Cost-sensitive classifiers that attempt to minimize these application-based penalties are needed. We propose a robust minimax approach for producing classifiers that directly minimize the cost of mistakes as a convex optimization problem. This is in contrast to previous methods that minimize the empirical risk using a convex surrogate for the cost of mistakes, since minimizing the empirical risk of the actual cost-sensitive loss is generally intractable. By treating properties of the training data as being uncertain, our approach avoids these computational difficulties. We develop theory and algorithms for our approach and demonstrate its benefits on cost-sensitive classification tasks.
Kaiser Asif, U of Illinois at Chicago; Wei Xing, U of Illinois at Chicago; Sima Behpour, U of Illinois at Chicago; Brian Ziebart, University of Illinois at Chicago
ID: 35 (pdf)
Intelligent Affect: Rational Decision Making for Socially Aligned Agents +
Affect Control Theory (ACT) is a mathematical model that makes accurate predictions about human behaviour across a wide range of settings. The predictions, which are derived from statistics about human actions and identities in real and laboratory environments, are shared prescriptive and affective behaviours that are believed to lead to solutions to everyday cooperative problems. A generalisation of ACT, called BayesACT, allows the principles of ACT to be used for human-interactive agents by combining a probabilistic version of the ACT dynamical model of affect with a utility function encoding external goals. Planning in BayesACT, which we address in this paper, then allows one to go beyond the affective prescription, and leads to the emergence of more complex interactions between ``cognitive'' reasoning and ``affective'' reasoning, such as deception leading to manipulation and altercasting. We use a continuous variant of a successful Monte-Carlo tree search planner (POMCP), which performs dynamic discretisation of the action and observation spaces while planning. We present results on two classic two-person social dilemmas, and show how reasoning about affect can produce some remarkably powerful, yet human-like, strategies in these games.
Nabiha Asghar, University of Waterloo; Jesse Hoey, University of Waterloo
ID: 37 (pdf)
Are You Doing What I Think You Are Doing? Criticising Uncertain Agent Models +
The key for effective interaction in many multiagent applications is to reason explicitly about the behaviour of other agents, in the form of a hypothesised behaviour. While there exist several methods for the construction of a behavioural hypothesis, there is currently no universal theory which would allow an agent to contemplate the correctness of a hypothesis. In this work, we present an novel algorithm which decides this question in the form of a frequentist hypothesis test. The algorithm allows for multiple metrics in the construction of the test statistic and learns its distribution during the interaction process, with asymptotic correctness guarantees. We present results from a comprehensive set of experiments, demonstrating that the algorithm achieves high accuracy and scalability at low computational costs.
Stefano Albrecht, The University of Edinburgh; Subramanian Ramamoorthy, The University of Edinburgh
ID: 38 (pdf)
Finite-Sample Analysis of GTD Algorithms +
In this paper, we conduct the finite-sample analysis of the gradient temporal difference learning (GTD) family of algorithms. Previous analyses of this class of algorithms use ODE techniques to show their asymptotic convergence, and to the best of our knowledge, no finite-sample analysis has been done. Moreover, there has been very little sample complexity analysis for reinforcement learning algorithms in off-policy learning scenarios. In this paper, we formulate the GTD methods as stochastic gradient algorithms w.r.t.~a primal-dual saddle-point objective function, and then conduct a saddle-point error bound analysis to obtain finite-sample error bounds of GTD algorithms family. Two revised algorithms are also proposed as projected GTD2 and GTD2-MP for better convergence guarantee and acceleration, respectively. The results of our theoretical analysis show that the GTD algorithms are indeed comparable to the existing LSTD methods in off-policy learning scenarios.
Bo Liu, University of Massachusetts Am; Ji Liu, University of Rochester; Mohammad Ghavamzadeh, Researcher / Chargé de Recherche (CR1), INRIA Lille - Team SequeL; Sridhar Mahadevan, School of Computer Science University of Massachusetts Amherst; Marek Petrik, IBM Research
ID: 41 (pdf)
Classification of Sparse and Irregularly Sampled Time Series with Mixtures of Expected Gaussian Kernels and Random Features +
This paper presents a kernel-based framework for classification of sparse and irregularly sampled time series. The properties of such time series can result in substantial uncertainty about the values of the underlying temporal processes, while making the data difficult to deal with using standard classification methods that assume fixed-dimensional feature spaces. To address these challenges, we propose to first re-represent each time series through the Gaussian process (GP) posterior it induces under a GP regression model. We then define kernels over the space of GP posteriors and apply standard kernel-based classification. Our primary contributions are (i) the development of a kernel between GPs based on the mixture of kernels between their finite marginals, (ii) the development and analysis of extensions of random Fourier features for scaling the proposed kernel to large-scale data, and (iii) an extensive empirical analysis of both the classification performance and scalability of our proposed approach.
Steven Cheng-Xian Li, UMass Amherst; Benjamin Marlin, UMass Amherst
ID: 46 (pdf)
Extend Transferable Belief Models with Probabilistic Priors +
In this paper, we extend Smets' transferable belief model (TBM) with probabilistic priors. Our first motivation for the extension is about evidential reasoning when the underlying prior knowledge base is Bayesian. We extend standard Dempster models with prior probabilities to represent beliefs and distinguish between two types of induced mass functions on an extended Dempster model: one for believing and the other essentially for decision-making. There is a natural correspondence between these two mass functions. In the extended model, we propose two conditioning rules for evidential reasoning with probabilistic knowledge base. Our second motivation is about the partial dissociation of betting at the pignistic level from believing at the credal level in TBM. In our extended TBM, we coordinate these two levels by employing the extended Dempster model to represent beliefs at the credal level. Pignistic probabilities are derived not from the induced mass function for believing but from the one for decision-making in the model and hence need not reply on the choice of frame of discernment. Moreover, we show that the above two proposed conditionings and marginalization (or coarsening) are consistent with pignistic transformation in the extended TBM.
Chunlai Zhou, Renmin University of China; Yuan Feng, University of Technology, Sydney
ID: 47 (pdf)
Population Empirical Bayes +
Bayesian predictive inference employs a model to analyze a dataset and make predictions about new observations. When a model does not match the data, predictive accuracy suffers. We develop population empirical Bayes, a hierarchical framework that explicitly models the empirical population distribution as part of Bayesian analysis. We introduce a latent dataset as a hierarchical variable and set the empirical population as its prior. This leads to a new predictive density that mitigates model mismatch. We efficiently apply this method to complex models by proposing a stochastic variational inference algorithm, called bumping variational inference. We demonstrate improved predictive accuracy over classical Bayesian inference in three models: a linear regression model of health data, a Bayesian mixture model of natural images, and a latent Dirichlet allocation topic model of a text corpus.
Alp Kucukelbir, Columbia University; David Blei, Columbia University
ID: 55 (pdf)
Computing Optimal Bayesian Decisions for Rank Aggregation via MCMC Sampling +
We propose two efficient and general MCMC algorithms to compute optimal Bayesian decisions for Mallows' model and Condorcet's model w.r.t.~any loss function and prior. We show that the mixing time of our Markov chain for Mallows' model is polynomial in $\varphi^{-k_{max}}$, $d_{max}$, and the input size, where $\varphi$ is the dispersion of the model, $k_{max}$ measures agents' largest total bias in bipartitions of alternatives, and $d_{max}$ is the maximum ratio between prior probabilities. We also show that in some cases the mixing time is at least $\Theta(\varphi^{-k_{max}/2})$. For Condorcet's model, our Markov chain is rapid mixing for moderate prior distributions. Efficiency of our algorithms are illustrated by experiments on real-world datasets.
David Hughes, RPI; Kevin Hwang, RPI; Lirong Xia, Rensselaer Polytechnic Institute
ID: 57 (pdf)
Incremental region selection for mini-bucket elimination bounds +
Region choice is a key issue for many approximate inference bounds. Mini-bucket elimination avoids the space and time complexity of exact inference by using a top-down partitioning approach that mimics the construction of a junction tree and aims to minimize the number of regions subject to a bound on their size; however, these methods rarely take into account functions’ values. In contrast, message passing algorithms often use “cluster pursuit” methods to select regions, a bottom-up approach in which a predefined set of clusters (such as triplets) is scored and incrementally added. In this work, we develop a hybrid approach that balances the advantages of both perspectives, providing larger regions chosen in an intelligent, energy-based way. Our method is applicable to bounds on a variety of inference tasks, and we demonstrate its power empirically on a broad array of problem types.
Sholeh Forouzan, UC, Irvine; Alexander Ihler, UC Irvine
ID: 58 (pdf)
(Nearly) Optimal Differentially Private Stochastic Multi-Arm Bandits +
We study the problem of private stochastic multi-arm bandits. Our notion of privacy is the same as some of the earlier works in the general area of private online learning [13, 17, 24]. We design algorithms that are i) differentially private, and ii) have regret guarantees that (almost) match the regret guarantees for the best non-private algorithms (e.g., upper confidence bound sampling and Thompson sampling). Moreover, through our experiments on both simulated and real datasets, we empirically show the effectiveness of our algorithms.
Nikita Mishra, 1989; Abhradeep Thakurta, Yahoo!
ID: 62 (pdf)
Parameterizing the Distance Distribution of Undirected Networks +
Network statistics such as node degree distributions, average path lengths, diameters, or clustering coefficients are widely used to characterize networks. One statistic that received considerable attention is the distance distribution --- the number of pairs of nodes for each shortest-path distance --- in undirected networks. determined therefrom; on the other hand, they are closely related to the dynamics of network spreading processes. It captures important properties of the network, reflecting on the dynamics of network spreading processes, and incorporates parameters such as node centrality and (effective) diameter. So far, however, no parameterization of the distance distribution is known that applies to a large class of networks. Here we develop such a closed-form distribution by applying maximum entropy arguments to derive a general, physically plausible model of path length histograms. Based on the model, we then establish the generalized Gamma as a three-parameter distribution for shortest-path distance in stronlgy-connected, undirected networks. Extensive experiments corroborate our theoretical results, which thus provide new approaches to network analysis.
Christian Bauckhage, Fraunhofer IAIS and University of Bonn; Kristian Kersting, TU Dortmund University; Fabian Hadiji, TU Dortmund University
ID: 67 (pdf)
Planning under Uncertainty with Weighted State Scenarios +
Decision making under uncertainty requires reasoning about uncertain events that may occur in the future. In many planning domains external factors are hard to model using a compact Markovian state. However, multiple consecutive state observations received from an environment may be correlated over time, which can be exploited during planning. In this paper we propose a scenario representation which enables agents to reason about sequences of future states. We show how weights can be assigned to scenarios, representing the likelihood that scenarios predict future states. Furthermore, we present a model based on a Partially Observable Markov Decision Process (POMDP), which can be used to reason about state scenarios during planning. In experiments we show how our scenario representation and POMDP model can be applied in the context of smart grids and stock markets. In both domains our model outperforms other methods that do not account for long term correlations.
Erwin Walraven, Delft University of Technology; Matthijs Spaan, Delft University of Technology
ID: 72 (pdf)
Bayes Optimal Feature Selection for Supervised Learning with General Performance Measures +
The problem of feature selection is critical in several areas of machine learning and data analysis. Here we consider feature selection for supervised learning problems, where one wishes to select a small set of features that facilitate learning a good prediction model in the reduced feature space. Our interest is primarily in filter methods that select features independently of the learning algorithm to be used and are generally faster to implement than wrapper methods. Many common filter methods for feature selection make use of mutual information based criteria to guide their search process. However, even in simple binary classification problems, mutual information based methods do not always select the best set of features in terms of the Bayes error. In this paper, we develop a filter method that directly aims to select the optimal set of features for a general performance measure of interest. Our approach uses the Bayes error with respect to the given performance measure as the criterion for feature selection and applies a greedy algorithm to optimize this criterion. We demonstrate application of this method to a variety of learning problems involving different performance measures. Experiments suggest the proposed approach is competitive with several state-of-the-art methods.
Saneem Ahmed CG, IBM India Research Lab; Harikrishna Narasimhan, Indian Institute of Science; Shivani Agarwal, Indian Institute of Science
ID: 73 (pdf)
Annealed Gradient Descent for Deep Learning +
In this paper, we propose a novel annealed gradient descent (AGD) method for deep learning. AGD optimizes a sequence of gradually improved smoother mosaic functions that approximate the original non-convex objective function according to an annealing schedule during optimization process. We present a theoretical analysis on its convergence properties and learning speed. The proposed AGD algorithm is applied to learning deep neural networks (DNN) for image recognition in MNIST and speech recognition in Switchboard. Experimental results have shown that AGD can yield comparable performance as SGD but it can significantly expedite training of DNNs in big data sets (by about 40% faster).
Hengyue Pan, York University; Hui Jiang, York University
ID: 77 (pdf)
Discriminative Switching Linear Dynamical Systems applied to Physiological Condition Monitoring +
We present a Discriminative Switching Linear Dynamical System (DSLDS) applied to patient monitoring in Intensive Care Units (ICUs). Our approach is based on identifying the state-of-health of a patient given their observed vital signs using a discriminative classifier, and then inferring their underlying physiological values conditioned on this status. The work builds on the Factorial Switching Linear Dynamical System (FSLDS) (Quinn et al., 2009) which has been previously used in a similar setting. The FSLDS is a generative model, whereas the DSLDS is a discriminative model. We demonstrate on two real-world datasets that the DSLDS is able to outperform the FSLDS in most cases of interest, and that an alpha-mixture of the two models achieves higher performance than either of the two models separately.
Konstantinos Georgatzis, University of Edinburgh; Christopher Williams, University of Edinburgh
ID: 81 (pdf)
Progressive Abstraction Refinement for Sparse Sampling +
Monte Carlo tree search (MCTS) algorithms can encounter difficulties when solving Markov decision problems (MDPs) in which the outcomes of actions are highly stochastic. This stochastic branching can be reduced through state abstraction. In online planning with a time budget, there is a complex tradeoff between the loss in performance due to overly coarse abstraction versus the gain in performance from reducing the problem size. We find empirically that very coarse and unsound abstractions often outperform sound abstractions for practical planning budgets. Motivated by this, we propose a progressive abstraction refinement algorithm that refines an initially coarse abstraction during search in order to match the abstraction granularity to the sample budget. Our experiments demonstrate the strong performance of search with coarse abstractions, and show that our proposed algorithm combines the benefits of coarse abstraction at small sample budgets with the ability to exploit larger budgets for further performance gains.
Jesse Hostetler, Oregon State University; Alan Fern, Oregon State University; Thomas Dietterich, Oregon State University
ID: 83 (pdf)
Learning the Structure of Sum-Product Networks via an SVD-based Algorithm +
Sum-product networks (SPNs) are a recently developed class of deep probabilistic models where inference is tractable. We present two new structure learning algorithms for sum-product networks, in the generative and discriminative settings, that are based on recursively extracting rank-one submatrices from data. The proposed algorithms find the subSPNs that are the most coherent jointly in the instances and variables -- that is, whose instances are most strongly correlated over the given variables. Experimental results show that SPNs learned using the proposed generative algorithm have better likelihood and inference results -- is also much faster than -- than previous approaches. Finally, we apply the discriminative SPN structure learning algorithm to handwritten digit recognition tasks, where it achieves state-of-the-art performance for an SPN.
Tameem Adel, Radboud University Nijmegen; David Balduzzi, Victoria University of Wellington; Ali Ghodsi, University of Waterloo
ID: 86 (pdf)
Learning the Structure of Causal Models with Relational and Temporal Dependence +
Many real-world domains are inherently relational and temporal---they consist of heterogeneous entities that interact with each over time. Effective reasoning about causality in such domains requires representations that explicitly model relational and temporal dependence. In this work, we provide a formalization of temporal relational models. We define temporal extensions to abstract ground graphs---a lifted representation that abstracts paths of dependence over all possible ground graphs. Temporal abstract ground graphs enable a sound and complete method for answering d-separation queries on temporal relational models. These methods provide the foundation for a constraint-based algorithm, TRCD, that learns causal models from temporal relational data. We provide experimental evidence that demonstrates the need to explicitly represent time when inferring causal dependence. We also demonstrate the expressive gain of TRCD compared to earlier algorithms that do not explicitly represent time.
Katerina Marazopoulou, University of Massachusetts Am; Marc Maier, University of Massachusetts Amherst; David Jensen, University of Massachusetts Amherst
ID: 89 (pdf)
On the Computability of AIXI +
How could we solve the machine learning and the artificial intelligence problem if we had infinite computation? Solomonoff induction and the reinforcement learning agent AIXI are proposed answers to this question. Both are known to be incomputable. In this paper, we quantify this using the arithmetical hierarchy, and prove upper and corresponding lower bounds for incomputability. We show that AIXI is not limit computable, thus it cannot be approximated using finite computation. Our main result is a limit-computable ε-optimal version of AIXI with infinite horizon that maximizes expected rewards.
Jan Leike, ANU; Marcus Hutter, The Australian National University
ID: 91 (pdf)
Bethe Projections for Non-Local Inference +
Many inference problems in structured prediction are naturally solved by augmenting a tractable dependency structure with complex, non-local auxiliary objectives. This includes the mean field family of variational inference algorithms, soft- or hard-constrained inference using Lagrangian relaxation or linear programming, collective graphical models, and forms of semi-supervised learning such as posterior regularization. We present a method to discriminatively learn broad families of inference objectives, capturing powerful non-local statistics of the latent variables, while maintaining tractable and provably fast inference using non-Euclidean projected gradient descent with a distance-generating function given by the Bethe entropy. We demonstrate the performance and flexibility of our method by (1) extracting structured citations from research papers by learning soft global constraints, (2) achieving state-of-the-art results on a widely-used handwriting recognition task using a novel learned non-convex inference procedure, and (3) providing a fast and highly scalable algorithm for the challenging problem of inference in a collective graphical model applied to bird migration.
Luke Vilnis, UMass Amherst; David Belanger, UMass Amherst; Daniel Sheldon, UMass Amherst; Andrew McCallum, UMass Amherst
ID: 96 (pdf)
Improved Asymmetric Locality Sensitive Hashing (ALSH) for Maximum Inner Product Search (MIPS) +
Recently it was shown that the problem of Maximum Inner Product Search (MIPS) is efficient and it admits provably sub-linear hashing algorithms. In~\cite{Proc:Shrivastava_NIPS14}, the authors use asymmetric transformations which convert the problem of approximate MIPS into the problem of approximate near neighbor search which can be efficiently solved using L2-LSH. In this work, we revisit the problem of MIPS and argue that the quantizations used in L2-LSH is suboptimal for MIPS compared to signed random projections (SRP) which is another popular hashing scheme for cosine similarity (or correlations). Based on this observation, we provide different asymmetric transformations which convert the problem of approximate MIPS into the problem amenable to SRP instead of L2-LSH. An additional advantage of our scheme is that we also obtain LSH type space partitioning which is not possible with the existing scheme. Our theoretical analysis show that the new scheme is significantly better than the original scheme for MIPS. Experimental evaluations strongly support the theoretical findings. We also provide the first empirical comparison that shows the superiority of hashing over tree based methods~\cite{Proc:Ram_KDD12} for MIPS.
Anshumali Shrivastava, Cornell University; Ping Li, Rutgers University
ID: 97 (pdf)
A Finite Population Likelihood Ratio Test of the Sharp Null Hypothesis for Compliers +
In a randomized experiment with noncompliance, scientific interest is often in testing whether the treatment exposure X has an effect on the final outcome Y. We propose a finite-population significance test of the sharp null hypothesis that X has no effect on Y, within the principal stratum of compliers, using a generalized likelihood ratio test. We present a new algorithm that solves the corresponding integer programs.
Wen Wei Loh, University of Washington; Thomas Richardson, University of Washington
ID: 102 (pdf)
Optimal Threshold Control for Energy Arbitrage with Degradable Battery Storage +
Energy arbitrage has the potential to make electric grids more efficient and reliable. Batteries hold great promise for energy storage in arbitrage but can degrade rapidly with use. In this paper, we analyze the impact of storage degradation on the structure of optimal policies in energy arbitrage. We derive properties of the battery degradation response that are sufficient for the existence of optimal threshold policies, which are interpretable and relatively easy to compute. Our experimental results indicate that explicitly considering battery degradation in optimizing energy arbitrage significantly improves solution quality.
Marek Petrik, IBM; Xiaojian Wu, UMASS
ID: 103 (pdf)
Disciplined Convex Stochastic Programming: A New Framework for Stochastic Optimization +
We introduce disciplined convex stochastic programming (DCSP), a modeling framework that can significantly lower the barrier for modelers to specify and solve convex stochastic optimization problems, by allowing modelers to naturally express a wide variety of convex stochastic programs in a manner that reflects their underlying mathematical representation. DCSP allows modelers to express expectations of arbitrary expressions, partial optimizations, and chance constraints across a wide variety of convex optimization problem families (e.g., linear, quadratic, second order cone, and semidefinite programs). We illustrate DCSP's expressivity through a number of sample implementations of problems drawn from the operations research, finance, and machine learning literatures.
Alnur Ali, Carnegie Mellon University; J. Zico Kolter, Carnegie Mellon University; Steven Diamond, Stanford University; Stephen Boyd, Stanford University
ID: 105 (pdf)
Structure Learning Constrained by Node-Specific Degree Distribution +
We consider the problem of learning the structure of a Markov Random Field (MRF) when the node-specific degree distribution is provided. The problem setting is inspired by protein contact map prediction in which residue-specific contact number distribution can be estimated without predicting individual contacts beforehand. We replace the widely used l_1 regularization with a node-specific regularization derived from the predicted degree distribution and optimize the objective function using an Iterative Maximum Cost Bipartite Matching algorithm. When a node is predicted to have k edges, its largest k regularization coefficients are reduced, promoting appearance of k edges for that node. We predict node-specific degree distribution using multiple 2nd-order Conditional Neural Fields integrating both local and global information of a protein. Experimental results show that for protein contact prediction our approach yields a significant accuracy improvement when the predicted contact number is reasonably good.
Jianzhu Ma, TTIC; Qingming Tang, TTIC; Sheng Wang, TTIC; Feng Zhao, TTIC; Jinbo Xu, TTIC
ID: 106 (pdf)
Max-Product Belief Propagation for Linear Programming: Applications to Combinatorial Optimization +
Max-product belief propagation (BP) is a popular message-passing algorithm for computing a maximum a-posteriori (MAP) assignment in a joint distribution represented by a graphical model (GM). It has been shown that BP can solve a few classes of Linear Programming (LP) formulations to combinatorial optimization problems including maximum weight matching and shortest path, i.e., BP can be a distributed solver for certain LPs. However, those LPs and corresponding BP analysis are very sensitive to underlying problem setups, and it has been not clear what extent these results can be generalized to. In this paper, we obtain a generic criteria that BP converges to the optimal solution of given LP, and show that it is satisfied in LP formulations associated to many classical combinatorial optimization problems including maximum weight perfect matching, shortest path, traveling salesman, cycle packing and vertex cover. More importantly, our criteria can guide the BP design to compute fractional LP solutions, while most prior results focus on integral ones. Our results provide new tools on BP analysis and new directions on efficient solvers for large-scale LPs.
Sejun Park, KAIST; Jinwoo Shin, KAIST
ID: 107 (pdf)
How matroids occur in the context of learning Bayesian network structure +
In this paper we show that any connected matroid having a non-trivial cluster of BN variables as its ground set induces a facet-defining inequality for the polytope(s) used in the ILP approach to optimal BN structure learning. Our result applies to well-known k-cluster inequalities, which play a crucial role in the ILP approach.
Milan Studeny, Inst. Info. Theory and Autom.
ID: 109 (pdf)
Visual Causal Feature Learning +
We provide a rigorous definition of the visual cause of a behavior that is broadly applicable to the visually driven behavior in humans, animals, neurons, robots and other perceiving systems. Our framework generalizes standard accounts of causal learning to settings in which the causal variables need to be constructed from micro-variables. We prove the Causal Coarsening Theorem, which allows us to gain causal knowledge from observational data with minimal experimental effort. The theorem provides a connection to standard inference techniques in machine learning that identify features of an image that correlate with, but may not cause, the target behavior. Finally, we propose an active learning scheme to learn a manipulator function that performs optimal manipulations on the image to automatically identify the visual cause of a target behavior. We illustrate our inference and learning algorithms in experiments based on both synthetic and real data.
Krzysztof Chalupka, Caltech; Pietro Perona, Caltech; Frederick Eberhardt, Caltech
ID: 111 (pdf)
The Limits of Knowledge Compilation for Exact Model Counting +
We show limits on the efficiency of using current knowledge compilation techniques to make exact probabilistic inference for large classes of natural problems. In particular we show lower bounds on knowledge compilation to SDD and DNNF forms. DNNF representations generalize current knowledge representations used for these problems, while SDD representations are an important recent subclass of DNNF representations whose use is becoming increasingly widespread. We give the first lower bound analysis of the complexity of SDD representations by relating SDD size to best-partition communication complexity. We use this relationship to prove exponential lower bounds on the SDD size for representing a large class of problems that occur naturally as queries over probabilistic databases. We use this to derive simple examples for which SDDs must be exponentially less concise than FBDDs (read-once branching programs). Another consequence is that SDDs are not qualitatively more concise than OBDDs for representing unions of conjunctive queries. Finally, we derive exponential lower bounds on the sizes of DNNF representations using a new quasipolynomial simulation of DNNFs by nondeterministic FBDDs.
Vincent Liew, University of Washington; Paul Beame, University of Washington
ID: 114 (pdf)
An Upper Bound on the Global Optimum in Parameter Estimation +
Learning graphical model parameters from incomplete data is a non-convex optimization problem. Iterative algorithms, such as Expectation Maximization (EM), can be used to get a local optimum solution. However, little is known about the quality of the learned local optimum, compared to the unknown global optimum. We exploit variables that are always observed in the dataset to get an upper bound on the global optimum which can give insight into the quality of the parameters learned by estimation algorithms.
Khaled Refaat, UCLA; Adnan Darwiche, UCLA
ID: 120 (pdf)
Estimating the Partition Function by Discriminance Sampling +
Importance sampling (IS) and its variant, annealed IS (AIS) have been widely used for estimating the partition function in graphical models, such as Markov random fields and deep generative models. However, IS tends to underestimate the partition function and is subject to high variance when the proposal distribution is more peaked than the target distribution. On the other hand, "reverse" versions of IS and AIS tend to overestimate the partition function, and degenerate when the target distribution is more peaked than the proposal distribution. In this work, we present a simple, general method that gives much more reliable and robust estimates than either IS (AIS) or reverse IS (AIS). Our method works by converting the estimation problem into a simple classification problem that discriminates between the samples drawn from the target and the proposal. We give extensive theoretical and empirical justification; in particular, we show that an annealed version of our method significantly outperforms both AIS and reverse AIS as proposed by Burda et al. (2015), which has been the state-of-the-art for likelihood evaluation in deep learning.
Qiang Liu, MIT; Jian Peng, UIUC; Alexander Ihler, UC Irvine; John Fisher III, MIT
ID: 121 (pdf)
Fast Relative-Error Approximation Algorithm for Ridge Regression +
Ridge regression is one of the most popular and effective regularized regression methods, and one case of particular interest is that the number of features $p$ is much larger than the number of samples $n$, i.e. $p \gg n$. In this case, the standard optimization algorithm for ridge regression computes the optimal solution $\mathbf{x}^*$ in $O(n^2 p+n^3)$ time. In this paper, we propose a fast relative-error approximation algorithm for ridge regression. More specifically, our algorithm outputs a solution $\tilde\x$ satisfying $\|\tilde\x -\mathbf{x}^*\|_2 \le \epsilon\|\mathbf{x}^*\|_2$ with high probability and runs in $\tilde O(\nnz(\mathbf{A})+n^3/\epsilon^2)$ time, where $\nnz(\mathbf{A})$ is the number of non-zero entries of matrix $\mathbf{A}$. To the best of our knowledge, this is the first algorithm for ridge regression that runs in $o(n^2 p)$ time with provable relative-error approximation bound on the output vector. In addition, for supplements to our main result, we analyze the risk inflation bound of our algorithm and apply our techniques to two generalizations of ridge regression, including multiple response ridge regression and a non-linear ridge regression problem. Finally, we show empirical results on both synthetic and real datasets.
Shouyuan Chen, CUHK; Yang Liu, The Chinese University of Hong Kong; Michael Lyu, Chinese University of Hong Kong; Irwin King, The Chinese University of Hong Kong; Shengyu Zhang, The Chinese University of Hong Kong
ID: 123 (pdf)
Generalization Bounds for Transfer Learning under Model Shift +
Transfer learning (sometimes also referred to as domain-adaptation) algorithms are often used when one tries to apply a model learned from a fully labeled source domain, to an unlabeled target domain, that is similar but not identical to the source. Previous work on covariate shift focuses on matching the marginal distributions on observations $X$ across domains while assuming the conditional distribution $P(Y|X)$ stays the same. Relevant theory focusing on covariate shift has also been developed. Recent work on transfer learning under model shift deals with different conditional distributions $P(Y|X)$ across domains with a few target labels, while assuming the changes are smooth. However, no analysis has been provided to say when these algorithms work. In this paper, we analyze transfer learning algorithms under the model shift assumption. Our analysis shows that when the conditional distribution changes, we are able to obtain a generalization error bound of $O(\frac{1}{\lambda_* \sqrt{n_l}})$ with respect to the labeled target sample size $n_l$, modified by the smoothness of the change ($\lambda_*$) across domains. Our analysis also sheds light on conditions when transfer learning works better than no-transfer learning (learning by labeled target data only). Furthermore, we extend the transfer learning algorithm from a single source to multiple sources.
Xuezhi Wang, Carnegie Mellon Univ.; Jeff Schneider, Carnegie Mellon Univ
ID: 127 (pdf)
Do-calculus when the True Graph is Unknown +
The basic task of causal discovery is to estimate the causal effect of some set of variables on another given a set of data. In this work, we bridge the gap between causal structure discovery and the do-calculus by proposing a method for the identification of causal effects on the basis of arbitrary (equivalence) classes of semi-Markovian causal models. The approach uses a general logical representation of the d-separation constraints obtained from a causal structure discovery algorithm, which can then be queried by procedures implementing the do-calculus inference for causal effects. We show that the method is more efficient than a determination of causal effects using a naive enumeration of graphs in the equivalence class. Moreover, the method is complete with regard to the identifiability of causal effects for settings, in which extant methods not assuming the true graph to be known, only offer incomplete results. The method is entirely modular and easily adapted for different background settings.
Antti Hyttinen, University of Helsinki; Frederick Eberhardt, Caltech; Matti Järvisalo, University of Helsinki
ID: 130 (pdf)
A Markov Game Model for Valuing Player Actions in Ice Hockey +
A variety of advanced statistics are used to evaluate player actions in the National Hockey League, but they fail to account for the context in which an action occurs or to look ahead to the long-term effects of an action. We apply the Markov Game formalism to develop a novel approach to valuing player actions that incorporates context and lookahead. Dynamic programming is used to learn Q-functions that quantify the impact of actions on goal scoring resp. penalties. Learning is based on a massive dataset that contains over 2.8M events in the National Hockey League. The impact of player actions is found to vary widely depending on the context, with possible positive and negative effects for the same action. We show that lookahead makes a substantial difference to the action impact scores. Players are ranked according to the aggregate impact of their actions. We compare this impact ranking with previous player metrics, such as plus-minus, total points, and salary.
Kurt Routley, Simon Fraser University; Oliver Schulte, Simon Fraser University
ID: 131 (pdf)
Impact of Learning Strategies on the Quality of Bayesian Networks: An Empirical Evaluation +
We present results from a empirical evaluation of the impact of Bayesian network structure learning strategies on the learned structures. In particular, we investigate how learning algorithms with different optimality guarantees compare in terms of the structural aspects and generalisability of the produced network structures. For example, in terms of generalization to unseen testing data, we show that local search algorithms often benefit from a tight constraint on the number of parents of variables in the networks, while exact approaches tend to benefit from looser parent restrictions. Overall, we find that learning strategies with weak optimality guarantees show good performs synthetic datasets, but, compared to exact approaches, perform poorly on the more ``real-world'' datasets. The exact approaches, which guarantee to find globally optimal solutions, consistently generalize well to unseen testing data, motivating further work on increasing the robustness and scalability of such algorithmic approaches to Bayesian network structure learning.
Brandon Malone, Max Planck Institute; Matti Järvisalo, University of Helsinki; Petri Myllymaki, Helsinki Institute for Information Technology
ID: 135 (pdf)
Clustered Sparse Bayesian Learning +
Many machine learning and signal processing tasks involve computing sparse representations using an overcomplete set of features or basis vectors, with compressive sensing-based applications a notable example. While traditionally such problems have been solved individually for different tasks, this strategy ignores strong correlations that may be present in real world data. Consequently there has been a push to exploit these statistical dependencies by jointly solving a series of sparse linear inverse problems. In the majority of the resulting algorithms however, we must a priori decide which tasks can most judiciously be grouped together. In contrast, this paper proposes an integrated Bayesian framework for both clustering tasks together and subsequently learning optimally sparse representations within each cluster. While probabilistic models have been applied previously to solve these types of problems, they typically involve a complex hierarchical Bayesian generative model merged with some type of approximate inference, the combination of which renders rigorous analysis of the underlying behavior virtually impossible. On the other hand, our model subscribes to concrete motivating principles that we carefully evaluate both theoretically and empirically. Importantly, our analyses take into account all approximations that are involved in arriving at the actual cost function to be optimized. Results on synthetic data as well as image recovery from compressive measurements show improved performance over existing methods.
Yu Wang, Cambridge University; David Wipf, Jeong Min Yun, Wei Chen, Ian Wassell
ID: 139 (pdf)
Efficient Algorithms for Bayesian Network Parameter Learning from Incomplete Data +
We propose a family of efficient algorithms for learning the parameters of a Bayesian network from incomplete data. Our approach is based on recent theoretical analyses of missing data problems, which utilize a graphical representation, called the missingness graph. In the case of MCAR and MAR data, this graph need not be explicit, and yet we can still obtain closed-form, asymptotically consistent parameter estimates, without the need for inference. When this missingness graph is explicated (based on background knowledge), even partially, we can obtain even more accurate estimates with less data. Empirically, we illustrate how we can learn the parameters of large networks from large datasets, which are beyond the scope of algorithms like EM (which require inference).
Guy Van den Broeck, Karthika Mohan, Arthur Choi, UCLA; Judea Pearl
ID: 141 (pdf)
Communication Efficient Coresets for Empirical Loss Minimization +
In this paper, we study the problem of empirical loss minimization with l2-regularization in distributed settings with significant communication cost. Stochastic gradient descent (SGD) and its variants are popular techniques for solving these problems in large-scale applications. However, the communication cost of these techniques is usually high, thus leading to considerable performance degradation. We introduce a novel approach to reduce the communication cost while retaining good convergence properties. The key to our approach is the construction of a small summary of the data, called coreset, at each iteration and solve an easy optimization problem based on the coreset. We present a general framework for analyzing coreset-based optimization and provide interesting insights into existing algorithms from this perspective. We then propose a new coreset construction and provide its convergence analysis for a wide class of problems that include logistic regression and support vector machines. We demonstrate the performance of our algorithm on real-world datasets and compare it against state-of-the-art algorithms.
Sashank Jakkam Reddi, Carnegie Mellon University; Barnabas Poczos, Alex Smola
ID: 143 (pdf)
Importance sampling over sets: a new probabilistic inference scheme +
Computing expectations in high-dimensional spaces is a key challenge in probabilistic inference and machine learning. Monte Carlo sampling, and importance sampling in particular, is one of the leading approaches. We propose a generalized importance sampling scheme based on randomly selecting (exponentially large) subsets of states rather than individual ones. By collecting a small number of extreme states in the sampled sets, we obtain estimates of statistics of interest, such as the partition function of an undirected graphical model. We incorporate this idea into a novel maximum likelihood learning algorithm based on cutting planes. We demonstrate empirically that our scheme provides accurate answers and scales to problems with up to a million variables.
Stefan Hadjis, Stanford University; Stefano Ermon, Stanford University
ID: 146 (pdf)
Novel Bernstein-like Concentration Inequalities for the Missing Mass +
We are concerned with obtaining novel concentration inequalities for the missing mass, i.e. the total probability mass of the outcomes not observed in the sample. We not only derive - for the first time - distribution-free Bernstein-like deviation bounds with sublinear exponents in deviation size for missing mass, but also improve the results of McAllester and Ortiz (2003) and Berend and Kontorovich (2013, 2012) for small deviations which is the most interesting case in learning theory. It is known that standard inequalities can not be used to analyze heterogeneous distributions i.e. distributions whose bins have large difference in magnitude. Our generic and intuitive approach shows that the heterogeneity issue introduced in McAllester and Ortiz(2003) is resolvable at least in the case of missing mass via regulating the terms using our novel thresholding technique.
Bahman Yari Saeed Khanloo, Monash; Gholamreza Haffari, Monash University
ID: 155 (pdf)
A Complete Generalized Adjustment Criterion +
Covariate adjustment is a widely used approach to estimate total causal effects from observational data. Several graphical criteria have been developed in recent years to identify valid covariates for adjustment from graphical causal models. These criteria can handle multiple causes, latent confounding, or partial knowledge of the causal structure; however, their diversity is confusing and some of them are only sufficient, but not necessary. In this paper, we present a criterion that is necessary and sufficient for four different classes of graphical causal models: directed acyclic graphs (DAGs), maximum ancestral graphs (MAGs), completed partially directed acyclic graphs (CPDAGs), and partial ancestral graphs (PAGs). Our criterion subsumes the existing ones and in this way unifies adjustment set construction for a large set of graph classes.
Emilija Perkovic, ETH Zurich; Johannes Textor, Utrecht University; Markus Kalisch, ETH Zurich; Marloes Maathuis, ETH Zurich
ID: 158 (pdf)
Locally Conditioned Belief Propagation +
Conditioned Belief Propagation (CBP) is an algorithm for approximate inference in probabilistic graphical models. It works by conditioning on a subset of variables, and solving the remainder using loopy Belief Propagation. Unfortunately, CBP's runtime scales exponentially in the number of conditioned variables. Locally Conditioned Belief Propagation (LCBP) approximates the results of CBP by treating conditions locally, and in this way avoids the exponential blow-up. We formulate LCBP as a variational optimization problem and derive a set of update equations that can be used to solve it. We show empirically that LCBP delivers results that are close to those obtained from CBP, while the computational cost scales favorably with problem size.
Thomas Geier, Ulm University; Felix Richter, Ulm University; Susanne Biundo, Ulm University
ID: 160 (pdf)
Optimal expert elicitation to reduce interval uncertainty +
Reducing uncertainty is an important problem in many applications such as risk and reliability analysis, system design, etc. In this paper, we study the problem of optimally querying experts to reduce interval uncertainty. Surprisingly, this problem has received little attention in the past, while similar issues in preference elicitation or social choice theory have witnessed a rising interest. We propose and discuss some solutions to determine optimal questions in a myopic way (one-at-a-time), and study the computational aspects of these solutions both in general and for some specific functions of practical interest. Finally, we illustrate the application of the approach in reliability analysis problems.
Nadia Ben Abdallah, Université de Technologie de C; Sébastien Destercke, Université de Technologie de Compiègne
ID: 165 (pdf)
Off-policy learning based on weighted importance sampling with linear computational complexity +
Importance sampling is an essential component of model-free off-policy learning algorithms. Weighted importance sampling (WIS) is generally considered superior to ordinary importance sampling but, when combined with function approximation, it has hitherto required computational complexity that is $O(n^2)$ or more in the number of features. In this paper we introduce new off-policy learning algorithms that obtain most of the benefits of WIS with $O(n)$ computational complexity. Our algorithms maintain for each component of the parameter vector a measure of the extent to which that component has been used in previous examples. This measure is used to determine component-wise step sizes, merging the ideas of stochastic gradient descent and sample averages. We present our main WIS-based algorithm first in an intuitive acausal form (the forward view) and then derive a causal algorithm using eligibility traces that is equivalent but more efficient (the backward view). In three small experiments, our algorithms performed significantly better than prior $O(n)$ algorithms for off-policy policy evaluation. We also show that our adaptive step-size technique alone can improve the performance of on-policy algorithms such as TD\la and true online TD\la.
Ashique Rupam Mahmood, University of Alberta; Richard Sutton, University of Alberta
ID: 167 (pdf)
State Sequence Analysis in Hidden Markov Models +
Given a discrete time finite state Hidden Markov Model (HMM) and a sequence of observations, different algorithms exist to answer different inference questions about the hidden states that the HMM traversed through. In this paper, the problem of finding the most probable state sequence is considered. The state sequence, as opposed to state trajectory, is a sequence of states that the HMM visited but without specifying the dwelling times in these states. This inference problem is relevant in a variety of domains, like text analysis, behavior recognition and etc. However, none of the existing algorithms addresses this inference question adequately. Previously, the problem of finding the most probable state sequence has been considered within the scope of continuous time Markov chains. Building on that work, we develop a provably correct algorithm, called \textit{state sequence analysis}, that addresses this inference question in HMMs. We discuss and illustrate empirically the differences between finding the most probable state sequence directly and doing so through running Viterbi algorithm and collapsing repetitive state visitations. Two synthetic experimental results demonstrate settings where Viterbi-based approach can be, at times significantly, suboptimal as compared to state sequence analysis. Further, we demonstrate the benefits of the proposed approach on a real activity recognition problem.
Yuri Grinberg, Ottawa Hospital Research Inst.; Theodore Perkins, Ottawa Hospital Research Institute
ID: 168 (pdf)
On the Error of Random Fourier Features +
Kernel methods give powerful, flexible, and theoretically well-understood approaches to solving many problems in machine learning. The standard approach, however, requires pairwise evaluations of a kernel function, which can lead to scalability issues for very large datasets. Rahimi and Recht (2007) suggested a popular approach to handling this problem, known as random Fourier features. The quality of this approximation, however, is not well-understood. We improve the uniform error bound of that paper, as well as giving novel understandings of the embedding's variance, approximation error, and use in some machine learning methods. We also point out that surprisingly, of the two main variants of those features, the more widely used is strictly higher-variance for the Gaussian kernel and has worse bounds.
Dougal Sutherland, Carnegie Mellon University; Jeff Schneider, Carnegie Mellon Univ
ID: 177 (pdf)
A Smart-Dumb/Dumb-Smart Algorithm for Efficient Split-Merge MCMC +
Split-merge moves are a standard component of MCMC algorithms for tasks such as multitarget tracking and fitting mixture models with unknown numbers of components. Achieving rapid mixing for split-merge MCMC has been notoriously difficult, and state-of-the-art methods do not scale well. We explore the reasons for this and propose a new split-merge kernel consisting of two sub-kernels: one combines a ``smart'' split move that proposes plausible splits of heterogeneous clusters with a ``dumb'' merge move that proposes merging random pairs of clusters; the other combines a dumb split move with a smart merge move. We show that the resulting smart-dumb/dumb-smart (SDDS) algorithm outperforms previous methods. Experiments with entity-mention models and Dirichlet process mixture models demonstrate much faster convergence and much better scaling to large data sets.
Wei WANG, UPMC; Stuart Russell
ID: 180 (pdf)
Encoding Markov logic networks in Possibilistic Logic +
Markov logic uses weighted formulas to compactly encode a probability distribution over possible worlds. Despite the use of logical formulas, Markov logic networks (MLNs) can be difficult to interpret, due to the often counter-intuitive meaning of their weights. To address this issue, we propose a method to construct a possibilistic logic theory that exactly captures what can be derived from a given MLN using maximum a posteriori (MAP) inference. Unfortunately, the size of this theory is exponential in general. We therefore also propose two methods which can derive compact theories that still capture MAP inference, but only for specific types of evidence. These theories can be used, among others, to make explicit the hidden assumptions underlying an MLN or to explain the predictions it makes.
Ondrej Kuzelka, Cardiff University; Jesse Davis, KU Leuven; Steven Schockaert, Cardiff University
ID: 187 (pdf)
Approximate Probabilistic Inference in Hybrid Domains by Hashing +
In the recent years, there has been considerable progress on fast randomized algorithms that approximate probabilistic inference with tight tolerance and confidence guarantees. The idea here is to formulate inference as a counting task over an annotated propositional theory, called weighted model counting (WMC), which can be partitioned into smaller tasks using universal hashing. An inherent limitation of this approach, however, is that it only admits the inference of discrete probability distributions. In this work, we consider the problem of approximating inference tasks for a probability distribution defined over discrete and continuous random variables. Building on a notion called weighted model integration, which is a strict generalization of WMC and is based on annotating Boolean and arithmetic constraints, we show how probabilistic inference in hybrid domains can be put within reach of hashing-based WMC solvers. Empirical evaluations demonstrate the applicability and promise of the proposal.
Vaishak Belle, KU Leuven; Guy Van den Broeck, KU Leuven; Andrea Passerini, University of Trento
ID: 188 (pdf)
Bayesian Network Learning with Discrete Case-Control Data +
We address the problem of learning Bayesian networks from discrete, unmatched case- control data using specialized conditional in- dependence tests. Those tests can also be used for learning other types of graphical models or for feature selection. We also propose a post-processing method that can be applied in conjunction with any Bayesian network learning algorithm. In simulations we show that our methods are able to deal with selection bias from case-control data.
Giorgos Borboudakis, University of Crete; Ioannis Tsamardinos, University of Crete
ID: 189 (pdf)
Learning Optimal Chain Graphs with Answer Set Programming +
Learning an optimal chain graph for a given probability distribution is an important but at the same time very hard computational problem. We present a new approach to solve this problem for various objective functions, and without making any assumption on the probability distribution at hand. Our approach is based on encoding the learning problem declaratively using the answer set programming (ASP) paradigm. Empirical results show that our approach provides at least as accurate solutions as the best solutions provided by the existing algorithms, and overall provides better accuracy than any single previous algorithm.
Dag Sonntag, Linköping University; Matti Järvisalo, University of Helsinki; Jose Pena, Linkoping University; Antti Hyttinen, University of Helsinki
ID: 190 (pdf)
Probabilistic Graphical Models Parameter Learning with Transferred Prior and Constraints +
Learning accurate Bayesian networks (BNs) is a key challenge in real-world applications, especially when training data are hard to acquire. Two approaches have been used to address this challenge: 1) introducing expert judgements and 2) transferring knowledge from related domains. This is the first paper to present a generic framework that combines both approaches to improve BN parameter learning. This framework is built upon an extended multinomial parameter learning model, that itself is an auxiliary BN. It serves to integrate both knowledge transfer and expert constraints. Experimental results demonstrate improved accuracy of the new method on a variety of benchmark BNs, showing its potential to benefit many real-world problems.
Yun Zhou, Queen Mary University of Londo; Norman Fenton, Queen Mary University of London; Timothy Hospedales, Queen Mary University of London; Martin Neil, Queen Mary University of London
ID: 191 (pdf)
Large-scale randomized-coordinate descent methods with non-separable linear constraints +
We develop randomized block coordinate de- scent (CD) methods for linearly constrained con- vex optimization. Unlike other large-scale CD methods, we do not assume the constraints to be separable, but allow them be coupled linearly. To our knowledge, ours is the first CD method that allows linear coupling constraints, without making the global iteration complexity have an exponential dependence on the number of con- straints. We present algorithms and theoreti- cal analysis for four key (convex) scenarios: (i) smooth; (ii) smooth + separable nonsmooth; (iii) asynchronous parallel; and (iv) stochastic. We discuss some architectural details of our methods and present preliminary results to illustrate the behavior of our algorithms.
Ahmed Hefny, Carnegie Mellon University; Sashank Jakkam Reddi, Carnegie Mellon University; Carlton Downey, Carnegie Mellon University; Avinava Dubey, Carnegie Mellon University; Suvrit Sra, MIT
ID: 202 (pdf)
Stable Spectral Learning Based on Schur Decomposition +
Spectral methods are a powerful tool for inferring the parameters of certain classes of probability distributions by means of standard eigenvalue-eigenvector decompositions. Spectral algorithms can be orders of magnitude faster than log-likelihood based and related iterative methods, and, thanks to the uniqueness of the spectral decomposition, they enjoy global optimality guarantees. In practice, however, the applicability of spectral methods is limited due to their sensitivity to model misspecification, which can cause instability issues in the case of non-exact models. We present a new spectral approach that is based on the Schur triangularization of a family of nearly-commuting matrices, and we carry out the corresponding theoretical analysis. Our main result is a theoretical bound on the estimation error, which is shown to depend directly on the model misspecification error and inversely on an eigenvalue separation gap. Numerical experiments show that the proposed method is more stable, and performs better in general, than the classical spectral approach based on direct matrix diagonalization.
Nikos Vlassis, Adobe; Nicolo Colombo, LCSB, Univ of Luxembourg
ID: 203 (pdf)
Budgeted Online Collective Inference +
Updating inference in response to new evidence is a fundamental challenge in artificial intelligence. Many real problems require large probabilistic graphical models, containing possibly millions of interdependent variables. For such large models, jointly updating the most likely (i.e., MAP) configuration of the variables each time new evidence is encountered can be infeasible, even if inference is tractable. In this paper, we explore budgeted online collective inference, in which the MAP configuration of a graphical model is updated efficiently by revising the assignments to a subset of the variables while holding others fixed. The goal is to selectively update certain variables without sacrificing quality with respect to full inference. To formalize the consequences of partially updating inference, we introduce the concept of inference regret. We derive inference regret bounds for a class of graphical models with strongly-convex free energies. These theoretical insights, combined with a thorough analysis of the optimization solver, motivate several new approximate methods for efficiently updating the variable assignments under a budget constraint. In experiments, we demonstrate that our algorithms can reduce inference time by 65\%, and with accuracy comparable to full inference.
Jay Pujara, University of Maryland; Ben London, University of Maryland; Lise Getoor, University of California, Santa Cruz
ID: 204 (pdf)
Missing Data as a Causal and Probabilistic Problem +
Causal inference is often phrased as a missing data problem -- for every unit, only the response to observed treatment assignment is known, the response to other treatment assignments is not. In this paper, we extend the converse approach of (Mohan et al, 2013) of representing missing data problems to restricted causal models (where only interventions on missingness indicators are allowed). We further use this representation to leverage techniques developed for the problem of identification of causal effects to give a general criterion for cases where a joint distribution containing missing variables can be recovered from data actually observed, given assumptions on missingness mechanisms. This criterion is significantly more general than the commonly used ``missing at random'' (MAR) criterion, and generalizes past work which also exploits a graphical representation of missingness. In fact, the relationship of our criterion to MAR is not unlike the relationship between the ID algorithm for identification of causal effects (Tian and Pearl, 2002), (Shpitser and Pearl 2006), and conditional ignorability (Rosenbaum and Rubin, 1983).
Ilya Shpitser, University of Southampton; Karthika Mohan, UCLA; Judea Pearl, UCLA
ID: 208 (pdf)
Scalable Recommendation with Hierarchical Poisson Factorization +
We develop hierarchical Poisson matrix factorization (HPF), a novel method for providing users with high quality recommendations based on implicit feedback, such as views, clicks, or purchases. In contrast to existing recommendation models, HPF has a number of desirable properties. First, we show that HPF more accurately captures the long-tailed user activity found in most consumption data by explicitly considering the fact that users have finite attention budgets. This leads to better estimates of users' latent preferences, and therefore superior recommendations, compared to competing methods. Second, HPF learns these latent factors by only explicitly considering positive examples, eliminating the often costly step of generating artificial negative examples when fitting to implicit data. Third, HPF is more than just one method---it is the simplest in a class of probabilistic models with these properties, and can easily be extended to include more complex structure and assumptions. We develop a variational algorithm for approximate posterior inference for HPF that scales up to large data sets, and we demonstrate its performance on a wide variety of real-world recommendation problems---users rating movies, listening to songs, reading scientific papers, and reading news articles. Our study reveals that hierarchical Poisson factorization definitively outperforms previous methods, including nonnegative matrix factorization, topic models, and probabilistic matrix factorization techniques.
Prem Gopalan, Princeton University; Jake Hofman, Microsoft Research; David Blei, Columbia University
ID: 209 (pdf)
Large-Margin Determinantal Point Processes +
Determinantal point processes (DPPs) offer a powerful approach to modeling diversity in many applications where the goal is to select a diverse subset from a ground set of items. We study the problem of learning the parameters (i.e., the kernel matrix) of a DPP from labeled training data. In this paper, we develop a novel parameter estimation technique particularly tailored for DPPs based on the principle of large margin separation. In contrast to the state-of-the-art method of maximum likelihood estimation of the DPP parameters, our large-margin loss function explicitly models errors in selecting the target subsets, and it can be customized to trade off different types of errors (precision vs.~recall). Extensive empirical studies validate our contributions, including applications on challenging document and video summarization, where flexibility in balancing different errors while training the summarization models is indispensable.
Boqing Gong, University of Southern Califor; Wei-Lun Chao, USC; Kristen Grauman, U. of Texas at Austin; Fei Sha, USC
ID: 213 (pdf)
Psychophysical Testing with Bayesian Active Learning +
Psychophysical detection tests are ubiquitous in the study of human sensation and the diagnosis and treatment of virtually all sensory impairments. In many of these settings, the goal is to recover, from a series of binary observations from a human subject, the latent function that describes the discriminability of a sensory stimulus over some relevant domain. The auditory detection test, for example, seeks to understand a subject's likelihood of hearing sounds as a function of frequency and amplitude. Conventional methods for performing these tests involve testing stimuli on a pre-determined grid. This approach not only samples at very uninformative locations, but also fails to learn critical features of a subject's latent discriminability function. Here we advance active learning with Gaussian processes to the setting of psychophysical testing. We develop a model that incorporates strong prior knowledge about the class of stimuli, we derive a sensible method for choosing sample points, and we demonstrate how to evaluate this model efficiently. Finally, we develop a novel likelihood that enables testing of multiple stimuli simultaneously. We evaluate our method in both simulated and real auditory detection tests, demonstrating the merit of our approach.
Jacob Gardner, Washington University in St. L; Xinyu Song, Washington University in St. Louis; Kilian Weinberger, Washington University in St. Louis; John Cunningham, Columbia University; Dennis Barbour, Washington University in St. Louis
ID: 221 (pdf)
Learning from Pairwise Marginal Independencies +
Dependency graphs (also called association graphs or bidirected graphs) represent marginal independencies amongst a set of variables. We give a characterization of the directed acyclic graphs (DAGs) that faithfully explain a given dependency graph in terms of their transitive closures, and use it to efficiently enumerate such structures. Our results map out the space of faithful causal models for given marginal independence relations, and show to which extent causal inference is possible without using conditional independence tests.
Johannes Textor, Utrecht University; Alexander Idelberger, Universität zu Lübeck; Maciej Liskiewicz, Universität zu Lübeck
ID: 224 (pdf)
Estimating Mutual Information by Local Gaussian Approximation +
A common problem found in machine learning, data analysis, and statistics is the estimation of mutual information. Previous works have shown that non-parametric estimation of mutual information is more difficult for strongly dependent variables. We present Local Gaussian Approximation (LGA), a simple semi-parametric estimator of mutual information based on finite i.i.d. samples drawn from an unknown probability distribution. We estimate mutual information as a sample expectation of log density ratios. At each sample point, densities are locally approximated via Gaussians. We show the consistency of our method and demonstrate that, unlike existing methods, the new estimator is able to accurately measure relationship strengths over many orders of magnitude.
Shuyang Gao, USC; Greg Ver Steeg, Information Sciences Institute; Aram Galstyan, Information Sciences Institute
ID: 226 (pdf)
Semi-described and semi-supervised learning with Gaussian processes +
Propagating input uncertainty through non-linear Gaussian process (GP) mappings is intractable. This hinders the task of training GPs using uncertain and partially observed inputs. In this paper, we christen this task "semi-described learning". We then introduce a GP framework that solves both, the semi-described and the semi-supervised learning problem (where missing values occur in the outputs). Auto-regressive state space simulation is also recognised as a special case of semi-described learning. To achieve our goal, we develop variational methods for handling semi-described inputs in GPs, and couple them with algorithms that allow for imputing the missing values while treating the uncertainty in a principled, Bayesian manner. Extensive experiments on simulated and real-world data study the problems of iterative forecasting and regression/classification with missing values. The results suggest that the principled propagation of uncertainty stemming from our framework can significantly improve performance in these tasks.
Andreas Damianou, University of Sheffield; Neil Lawrence
ID: 227 (pdf)
Bayesian Structure Learning for Stationary Time Series +
While much work has explored probabilistic graphical models for independent data, less attention has been paid to time series. The goal in this setting is to determine conditional independence relations between entire time series, which for stationary series, are encoded by zeros in the inverse spectral density matrix. We take a Bayesian approach to structure learning, placing priors on (i) the graph structure and (ii) spectral matrices given the graph. We leverage a Whittle likelihood approximation and define a conjugate prior---the hyper complex inverse Wishart---on the complex-valued and graph-constrained spectral matrices. Due to conjugacy, we can analytically marginalize the spectral matrices and obtain a closed-form marginal likelihood of the time series given a graph. Importantly, our analytic marginal likelihood allows us to avoid inference of the complex spectral matrices themselves and places us back into the framework of standard (Bayesian) structure learning. In particular, combining this marginal likelihood with our graph prior leads to efficient inference of the time series graph itself, which we base on a stochastic search procedure, though any standard approach can be straightforwardly modified to our time series case. We demonstrate our methods on analyzing stock data and neuroimaging data of brain activity during various auditory tasks.
Alex Tank, University of Washington; Emily Fox, University of Washington; Nicholas Foti, University of Washington
ID: 228 (pdf)
Equitable Partitions of Concave Free Energies +
Recently, exploiting symmetries within variational inference has been algebraically formalized. With the exception of TRW for marginal inference, however, the framework resulted in approximate MAP algorithms only, based on equitable and orbit partitions of the graphical model. Here, we deepen our understandnig of it for marginal inference. Specifically, we show that a large class of concave free energies admits equitable partitions, of which orbit partitions are a special case, that can be exploited for lifting. Although already interesting on its own, we go one step further. We demonstrate that concave free energies can be reparameterized so that existing convergent algorithms can be used for lifted variational marginal inference without modification.
Martin Mladenov, TU Dortmund; Kristian Kersting, TU Dortmund University
ID: 230 (pdf)
Learning to generate via MMD optimization +
We consider learning to generate samples from an unknown distribution given i.i.d. data. In particular, learning is cast as optimizing a transport function to minimize a two-sample test statistic—informally speaking, a good transport function produces samples that cause a two-sample test to fail to reject the null hypothesis. As our objective function, we use an unbiased estimator of the maximum mean discrepancy that is the test statistic underlying the kernel two-sample test proposed by Gretton et al. (2012). We compare to recent proposals for learning generative models and give bounds on the generalization error incurred from optimizing the empirical MMD.
Gintare Karolina Dziugaite, University of Cambridge; Zoubin Ghahramani, Cambridge University; Daniel Roy, University of Toronto
ID: 231 (pdf)
Non-parametric Revenue Optimization for Generalized Second Price auctions. +
We present an extensive analysis of the key prob- lem of learning optimal reserve prices for gen- eralized second price auctions. We describe two algorithms for this task: one based on den- sity estimation, and a novel algorithm benefit- ting from solid theoretical guarantees and with a very favorable running-time complexity of O(nS log(nS)), where n is the sample size and S the number of slots. Our theoretical guar- antees are more favorable than those previously presented in the literature. Additionally, we show that even if bidders do not play at an equilibrium, our second algorithm is still well defined and minimizes a quantity of interest. To our knowl- edge, this is the first attempt to apply learning algorithms to the problem of reserve price optimization in GSP auctions.
Mehryar Mohri, NYU; Andres Munoz Medina, NYU
ID: 235 (pdf)
Kernel-Based Just-In-Time Learning for Passing Expectation Propagation Messages +
We propose an efficient nonparametric strategy for learning a message operator in expectation propagation (EP), which takes as input the set of incoming messages to a factor node, and produces an outgoing message as output. This learned operator replaces the multivariate integral required in classical EP, which may not have an analytic expression. We use kernel-based regression, which is trained on a set of probability distributions representing the incoming messages, and the associated outgoing messages. The kernel approach has two main advantages: first, it is fast, as it is implemented using a novel two-layer random feature representation of the input message distributions; second, it has principled uncertainty estimates, and can be cheaply updated online, meaning it can request and incorporate new training data when it encounters inputs on which it is uncertain. In experiments, our approach is able to solve learning problems where a single message operator is required for multiple, substantially different data sets (logistic regression for a variety of classification problems), where the ability to accurately assess uncertainty and to efficiently and robustly update the message operator are essential.
Wittawat Jitkrittum, University College London; Arthur Gretton, Gatsby unit, University College London; Nicolas Heess, S. M. Ali Eslami, Google DeepMind; Balaji Lakshminarayanan, Gatsby/University College London; Dino Sejdinovic, University of Oxford; Zoltan Szabo, Gatsby unit, University College London
ID: 239 (pdf)
Efficient Transition Probability Computation for Continuous-Time Branching Processes via Compressed Sensing +
Branching processes are a class of continuous-time Markov chains (CTMCs) with ubiquitous applications. A general difficulty in statistical inference under partially observed CTMC models arises in computing transition probabilities when the discrete state space is large or uncountable. Classical methods such as matrix exponentiation are infeasible for large or countably infinite state spaces, and sampling-based alternatives are computationally intensive, requiring a large integration step to impute over all possible hidden events. Recent work has successfully applied generating function techniques to computing transition probabilities for linear multitype branching processes. While these techniques often require significantly fewer computations than matrix exponentiation, they also become prohibitive in applications with large populations. We propose a compressed sensing framework that significantly accelerates the generating function method, decreasing computational cost up to a logarithmic factor by only assuming the probability mass of transitions is sparse. We demonstrate accurate and efficient transition probability computations in branching process models for hematopoiesis and transposable element evolution.
Jason Xu, University of Washington; Vladimir Minin, University of Washington
ID: 246 (pdf)
Survival Filter: A Latent Timeseries Model for Joint Survival Analysis +
Survival analysis is a core task in applied statistics, which models time-to-failure or time-to-event data. In the clinical domain, meaningful events can be the onset of different disease for a given patient. Because patients often have a wide range of diseases with complex interactions amongst them, it would be beneficial to model time to all diseases simultaneously. We propose and describe the survival filter model for this task, and apply it to a real-world, large dataset of longitudinal patient records. The model admits a scalable variational inference algorithm based on noisy gradients constructed from sampling the variational approximation. Experiments show that the survival filter model gives good predictive performance when compared to two baselines, and identifies clinically meaningful latent factors to represent diseases that co-occur in time.
Rajesh Ranganath, Princeton University; Adler Perotte, Columbia University; Noemie Elhadad, Columbia University; David Blei, Columbia University
ID: 248 (pdf)
High-Dimensional Stochastic Integration via Error-Correcting Codes +
We consider the task of summing a non-negative function f over a discrete set Ω, e.g., to compute the partition function of a graphical model. Ermon et al. have shown that, in a probabilistic approximate sense, summation can be reduced to maximizing f over random subsets of Ω defined by parity (XOR) constraints. Unfortunately, XORs with many variables are computationally problematic, while XORs with few variables have no guarantees. We introduce two ideas to address this problem, both motivated by the theory of error-correcting codes. The first is to maximize f over explicitly generated random affine subspaces of Ω, which is equivalent to unconstrained maximization of f over an exponentially smaller domain. The second idea, closer in spirit to the original approach, is to use systems of linear equations defining error-correcting codes. Even though the equations in such systems only contain O(1) variables each, their sets of solutions (codewords) have excellent statistical properties. By combining these ideas we achieve 100x or greater speedup over the original approach and, perhaps more importantly, levels of accuracy that were completely unattainable.
Dimitris Achlioptas, UCSC; Pei Jiang
ID: 249 (pdf)
Geometric Network Comparisons +
Network analysis has a crucial need for tools to compare networks and assess the significance of differences between networks. We propose a principled statistical approach to network comparison that approximates networks as probability distributions on negatively curved manifolds. We outline the theory, as well as implement the approach on simulated networks.
Dena Asta, Carnegie Mellon University; Cosma Shalizi, Carnegie Mellon University
ID: 252 (pdf)
Selective Greedy Equivalence Search: Finding Optimal Bayesian Networks Using a Polynomial Number of Score Evaluations +
We introduce Selective Greedy Equivalence Search (SGES), a restricted version of Greedy Equivalence Search (GES). SGES retains the asymptotic correctness of GES but, unlike GES, has polynomial performance guarantees. In particular, we show that when data are sampled independently from a distribution that is perfect with respect to a DAG $\Gr$ defined over the observable variables then, in the limit of large data, SGES will identify $\Gr$'s equivalence class after a number of score evaluations that is (1) polynomial in the number of nodes and (2) exponential in various complexity measures including maximum-number-of-parents, maximum-clique-size, and a new measure called {\em v-width} that is necessarily not larger---and potentially much smaller---than the other two. More generally, we show that for any hereditary and equivalence-invariant property $\Pi$ known to hold in $\Gr$, we retain the large-sample optimality guarantees of GES even if we ignore any GES deletion operator that results in a state for which $\Pi$ does not hold in the common-descendants subgraph.
Max Chickering, Microsoft Research; Chris Meek, Microsoft Research
ID: 258 (pdf)
A Probabilistic Logic for Reasoning about Uncertain Temporal Information +
The main goal of this work is to present the proof-theoretical and model-theoretical approach to a probabilistic logic which allows reasoning about temporal information. We extend both the language of linear time logic and the language of probabilistic logic, allowing statements like ``A will always hold"and ``the probability that A will hold in next moment is at least the probability that B will always hold," where A and B are arbitrary statements. We axiomatize this logic, provide corresponding semantics and prove that the axiomatization is sound and strongly complete. We show that the problem of deciding decidability is PSPACE-complete, no worse than that of linear time logic.
Dragan Doder, University of Luxembourg; Zoran Ognjanovic, Mathematical Institute, Serbian Academy of Sciences and Arts
ID: 266 (pdf)
Hamiltonian ABC +
Approximate Bayesian computation (ABC) is a powerful and elegant framework for performing inference in simulation-based models. However, due to the difficulty in scaling likelihood estimates, ABC remains useful for relatively low-dimensional problems. We introduce Hamiltonian ABC (HABC), a set of likelihood-free algorithms that apply recent advances in scaling Bayesian learning using Hamiltonian Monte Carlo (HMC) and stochastic gradients. We find that a small number forward simulations can effectively approximate the ABC gradient, allowing Hamiltonian dynamics to efficiently traverse parameter spaces. We also describe a new simple yet general approach of incorporating random seeds into the state of the Markov chain, further reducing the random walk behavior of HABC. We demonstrate HABC on several typical ABC problems, and show that HABC samples comparably to regular Bayesian inference using true gradients on a high-dimensional problem from machine learning.
Edward Meeds, University of Amsterdam; Max Welling, Robert Leenders
ID: 268 (pdf)
Memory-Efficient Symbolic Online Planning for Factored MDPs +
Factored Markov Decision Processes (MDP) are a de facto standard for compactly modeling sequential decision making problems with uncertainty. Offline planning based on symbolic operators exploits the factored structure of MDPs, but is memory intensive. We present new memory-efficient symbolic operators for online planning that effectively generalize experience. The soundness of the operators and convergence of the planning algorithms are shown followed by experiments that demonstrate superior scalability in benchmark problems.
Aswin Raghavan, Oregon State University; Prasad Tadepalli, Oregon State University; Alan Fern, Oregon State University; Roni Khardon, Tufts University
ID: 271 (pdf)
Bayesian Optimal Control of Smoothly Parameterized Systems +
We study Bayesian optimal control of a general class of smoothly parameterized Markov decision problems (MDPs). We propose a \textit{lazy} version of the so-called posterior sampling method, a method that goes back to Thompson and Strens, more recently studied by Osband, Russo and van Roy. While Osband et al. derived a bound on the (Bayesian) regret of this method for undiscounted total cost episodic, finite state and action problems, we consider the continuing, average cost setting with no cardinality restrictions on the state or action spaces. While in the episodic setting, it is natural to switch to a new policy at the episode-ends, in the continuing average cost framework we must introduce switching points explicitly and in a principled fashion, or the regret could grow linearly. Our lazy method introduces these switching points based on monitoring the uncertainty left about the unknown parameter. To develop a suitable and easy-to-compute uncertainty measure, we introduce a new ``average local smoothness'' condition, which is shown to be satisfied in common examples. Under this, and some additional mild conditions, we derive rate-optimal bounds on the regret of our algorithm. Our general approach allows us to use a single algorithm and a single analysis for a wide range of problems, such as finite MDPs or linear quadratic regulation, both being instances of smoothly parameterized MDPs. The effectiveness of our method is illustrated by means of a simulated example.
Yasin Abbasi-Yadkori, QUT; Csaba Szepesvari, University of Alberta
ID: 277 (pdf)
Learning and Planning with Timing Information in Markov Decision Processes +
We consider the problem of learning and planning in Markov decision processes with temporally extended actions represented in the options framework. We propose to use predictions about the duration of extended actions to represent the state and show that this leads to a compact predictive state representation model independent of the set of primitive actions. Then we develop a consistent and efficient spectral learning algorithm for such models. Using just the timing information to represent states allows for faster improvement in the planning performance. We illustrate our approach with experiments in both synthetic and robot navigation domains.
Pierre-Luc Bacon, McGill University; Borja Balle, McGill University; Doina Precup, McGill University
ID: 281 (pdf)
Online Bellman Residual Algorithms with Predictive Error Guarantees +
We establish a connection between optimizing the Bellman Residual and worst case long-term predictive error. In the online learning framework, learning takes place over a sequence of trials with the goal of predicting a future discounted sum of rewards. Our analysis shows that, together with a stability assumption, any no-regret online learning algorithm that minimizes Bellman error ensures small prediction error. No statistical assumptions are made on the sequence of observations, which could be non-Markovian or even adversarial. Moreover, the analysis is independent of the particular form of function approximation and the particular (stable) no-regret approach taken. Our approach thus establishes a broad new family of provably sound algorithms for Bellman Residual-based learning and provides a generalization of previous worst-case result for minimizing predictive error. We investigate the potential advantages of some of this family both theoretically and empirically on benchmark problems.
Wen Sun, Carnegie Mellon University; J. Andrew Bagnell, Carnegie Mellon University
ID: 285 (pdf)
Fast Algorithms for Learning with Long $N$-grams via Suffix Tree Based Matrix Multiplication +
This paper addresses the computational and statistical issues of learning with long -- and possibly all -- $N$-grams in a document corpus. We leverage the rich algebraic structure of $N$-gram matrices to provide a data structure which can store and multiply any $N$-gram matrix in memory and time that is, at worst, linear in the length of the corpus from which it is derived. As matrix-vector multiplication lies at the heart of most machine learning algorithms, our algorithm can speed up any learning procedure that uses $N$-gram features and has such structure. We also provide an efficient, linear running time and memory, framework that produces our data structure and screens $N$-gram features according to a multitude of statistical criteria. We demonstrate the performance of our algorithm on natural language and DNA sequence datasets; the computational and memory savings are substantial. Finally, we apply our framework to several large-scale sentiment analysis problems involving millions of reviews over gigabytes of text and show that higher-order $N$-grams can substantially improve performance.
Hristo Paskov, Stanford University; Trevor Hastie, Stanford; John Mitchell, Stanford
ID: 293 (pdf)
Robust reconstruction of causal graphical models based on conditional 2-point and 3-point information +
We report a novel network reconstruction method, which combines constraint-based and Bayesian frameworks to reliably reconstruct graphical models despite inherent sampling noise in finite observational datasets. The approach is based on an information theory result tracing back the existence of colliders in graphical models to negative conditional 3-point information between observed variables. This enables to confidently ascertain structural independencies in causal graphs, based on the ranking of their most likely contributing nodes with (significantly) positive conditional 3-point information. Starting from a complete undirected graph, dispensible edges are progressively pruned by iteratively `taking off' the most likely positive conditional 3-point information from the 2-point (mutual) information between each pair of nodes. The resulting network skeleton is then partially directed by orienting and propagating edge directions, based on the sign and magnitude of the conditional 3-point information of unshielded triples. This `3off2' network reconstruction approach is shown to outperform both constraint-based and Bayesian inference methods on a range of benchmark networks.
Herve Isambert, CNRS; Severine Affeldt, Institut Curie
ID: 294 (pdf)
Auxiliary Gibbs Sampling for Inference in Piecewise-Constant Conditional Intensity Models +
A piecewise-constant conditional intensity model (PCIM) is a non-Markovian model of temporal stochastic dependencies in continuous- time event streams. It allows efficient learning and forecasting given complete trajectories. However, no general inference algorithm has been developed for PCIMs. We propose an effective and efficient auxiliary Gibbs sampler for inference in PCIM, based on the idea of thinning for inhomogeneous Poisson processes. The sampler alternates between sampling a finite set of auxiliary virtual events with adaptive rates, and performing an efficient forward-backward pass at discrete times to generate samples. We show that our sampler can successfully perform inference tasks in both Markovian and non-Markovian models, and can be employed in Expectation-Maximization PCIM parameter estimation and structural learning with partially observed data.
Zhen Qin, UC-Riverside; Christian Shelton, UC-Riverside
ID: 298 (pdf)
Averaging of Decomposable Graphs by Dynamic Programming and Sampling +
We give algorithms for Bayesian learning of decomposable graphical models from complete data. We build on a recently proposed dynamic programming algorithm that finds optimal graphs of $n$ nodes in $O(4^n)$ time and $O(3^n)$ space (Kangas et al., NIPS 2014), and show how it can be turned into accurate averaging algorithms. Specifically, we show that certain marginals of the posterior distribution, like the posterior probability of an edge, can be computed in $O(n^3 3^n)$ time, provided that the prior over the graphs is of an appropriate form. To overcome some limitations of the exact approach, we also give sampling schemes that---using essentially no extra space---can draw up to $3^n$ independent graphs from the posterior in $O(n 4^n)$ time. Through importance sampling, this enables accurate Bayesian inference with a broader class of priors. Using benchmark datasets, we demonstrate the method's performance and the advantage of averaging over optimization when learning from little data.
Kustaa Kangas, University of Helsinki; Teppo Niinimäki, University of Helsinki; Mikko Koivisto, Helsinki Institute for Information Technology
ID: 300 (pdf)
Multi-Context Models for Reasoning under Partial Knowledge: Generative Process and Inference Grammar +
Arriving at the complete probabilistic knowledge of a domain, i.e., learning how all variables interact, is indeed a demanding task. In reality, settings often arise for which an individual merely possesses partial knowledge of the domain, and yet, is expected to give adequate answers to a variety of posed queries. That is, although precise answers to some queries, in principle, cannot be achieved, a range of plausible answers is attainable for each query given the available partial knowledge. In this paper, we propose the Multi-Context Model (MCM), a new graphical model to represent the state of partial knowledge as to a domain. MCM is a middle ground between Probabilistic Logic, Bayesian Logic, and Probabilistic Graphical Models. For this model we discuss: (i) the dynamics of constructing a contradiction-free MCM, i.e., to form partial beliefs regarding a domain in a gradual and probabilistically consistent way, and (ii) how to perform inference, i.e., to evaluate a probability of interest involving some variables of the domain.
Ardavan Salehi Nobandegani, McGill University ; Ioannis Psaromiligkos, McGill University
ID: 302 (pdf)
Mesochronal Structure Learning +
Standard time series structure learning algorithms assume that the measurement timescale is approximately the same as the timescale of the underlying (causal) system. In many scientific contexts, however, this assumption is violated: the measurement timescale can be substantially slower than the system timescale (i.e., many intermediate time series datapoints are missing). This assumption violation can lead to significant learning errors. In this paper, we provide a novel learning algorithm that can extract system-timescale structure given measurement data that undersample the underlying system. Substantial algorithmic optimizations were required to achieve computational tractability. We conclude by showing that the algorithm is highly reliable at extracting system-timescale structure from undersampled data.
Sergey Plis, The Mind Research Network; Jianyu Yang, the Mind Research Network; David Danks, Carnegie Mellon University
ID: 305 (pdf)
A Statistical Framework for Clustering Representation Learning +
We address the problem of communicating domain knowledge from a user to the designer of a clustering algorithm. We propose a protocol that is based on the user providing a clustering of a relatively small random sample of a data set. The algorithm designer then uses that sample to come up with a data representation so that $k$-means clustering under that representation results in a clustering (of the full data set) that is aligned with the user's clustering. We provide a formal statistical model for analyzing the sample complexity of learning a clustering representation with this paradigm. We then introduce a notion of capacity of a class of possible representations, in the spirit of the VC-dimension, showing that classes of representations that have finite such dimension can be successfully learned with sample size error bounds, and end our discussion with an analysis of that dimension for classes of representations induced by linear embeddings.
Hassan Ashtiani, University of Waterloo; Shai Ben-David, University of Waterloo
ID: 307 (pdf)
Active Search on Graphs using Sigma-Optimality +
Many modern information access problems involve highly complex patterns that cannot be handled by traditional keyword based search. Active Search is an emerging paradigm that helps users quickly find relevant information by efficiently collecting and learning from user feedback. We consider active search on graphs, where the nodes represent the set of instances users want to search over and the edges encode pairwise similarity among the instances. Existing active search algorithms are either short of theoretical guarantees or inadequate for graph data. Motivated by recent advances in active learning on graphs, namely the Sigma-optimality selection criterion, we propose new active search algorithms suitable for graphs with theoretical guarantees and demonstrate their effectiveness on several real-world datasets.
Yifei Ma, CMU; Tzu-Kuo Huang, Microsoft Research; Jeff Schneider, Carnegie Mellon Univ
ID: 308 (pdf)
Multitasking: Optimal Planning for Bandit Superprocesses +
A bandit superprocess is a decision problem composed from multiple independent Markov decision processes (MDPs), coupled only by the constraint that, at each time step, the agent may act in only one of the MDPs. Multitasking problems of this kind are ubiquitous in the real world, yet very little is known about them from a computational viewpoint, beyond the basic observation that optimal policies for the superprocess may prescribe actions that would be suboptimal for an MDP considered in isolation. (This observation implies that many applications of sequential decision analysis in practice are technically incorrect, since the decision problem being solved is typically part of a larger, unstated bandit superprocess.) The paper summarizes the state-of-the-art in the theory of bandit superprocesses and contributes a novel upper bound on the global value function of a bandit superprocess, defined in terms of a direct relaxation of the arms. The bound is equivalent to an existing bound (the Whittle integral) and so provides insight into an otherwise opaque formula. We provide an algorithm to compute this bound and use it to derive the first practical algorithms to select optimal actions in bandit superprocesses. The algorithm operates by repeatedly establishing dominance relations between actions using upper and lower bounds on action values. Experiments indicate that the algorithm's run-time compares very favorably to other possible algorithms designed for more general factored MDPs.
Dylan Hadfield-Menell, UC Berkeley; Stuart Russell
ID: 312 (pdf)
Revisiting Non-Progressive Influence Models: Scalable Influence Maximization in Social Networks +
Influence maximization in social networks has been studied extensively in computer science community for the last decade. However, almost all of the efforts have been focused on the progressive influence models, such as independent cascade (IC) and Linear threshold (LT) models, which cannot capture the reversibility of choices. In this paper, we present the Heat Conduction (HC) model which is a non-progressive influence model and has favorable real-world interpretations. Moreover, we show that HC unifies, generalizes, and extends the existing non-progressive models, such as the Voter model [1] and nonprogressive LT [2]. We then prove that selecting the optimal seed set of influential nodes is NP-hard for HC but by establishing the submodularity of influence spread, we can tackle the influence maximization problem with a scalable and provably near-optimal greedy algorithm. To the best of our knowledge, we are the first to present a scalable solution for influence maximization under non-progressive LT model, as a special case of HC model. In sharp contrast to the other greedy influence maximization methods, our fast and efficient C2GREEDY algorithm benefits from two analytically computable steps: closed-form computation for finding the influence spread as well as the greedy seed selection. Through extensive experiments on several and large real and synthetic networks, we show that C2GREEDY outperforms the state-of-the-art methods, under HC model, in terms of both influence spread and scalability.
Golshan Golnari, University of Minnesota; Amir Asiaee T., University of Minnesota; Arindam Banerjee, University of Minnesota; Zhi-Li Zhang, University of Minnesota
ID: 316 (pdf)
Learning Latent Variable Models via Method of Moments and Exterior Point Optimization +
Probabilistic latent-variable models are a fundamental tool in statistics and machine learning. Despite their widespread use, identifying the parameters of basic latent variable models continues to be an extremely challenging problem. Traditional maximum likelihood-based learning algorithms find valid parameters, but suffer from high computational cost, slow convergence, and local optima. In contrast, recently developed method of moments-based algorithms are computationally efficient and provide strong statistical guarantees, but are not guaranteed to find valid parameters. In this work, we introduce a two-stage learning algorithm for latent variable models. We first use method of moments to find a solution that is close to the optimal solution but not necessarily in the valid set of model parameters. We then incrementally refine the solution via exterior point optimization until a local optima that is arbitrarily near the valid set of parameters is found. We perform several experiments on synthetic and real-world data and show that our approach is more accurate then previous work, especially when training data is limited.
Amirreza Shaban, Georgia Institute of Technolog; Mehrdad Farajtabar, Georgia Institute of Technology; Bo Xie, Georgia Institute of Technology; Le Song, Georgia Tech; Byron Boots, Georgia Institute of Technology
ID: 319 (pdf)
Zero-Truncated Poisson Model for Scalable Bayesian Factorization of Massive Binary Tensors with Mode-Networks +
We present a scalable Bayesian model for low-rank factorization of massive tensors with binary observations. The proposed model has the following key properties: (1) in contrast to the models based on logistic or probit likelihood, using a zero-truncated Poisson likelihood for binary data allows our model to scale up in the number of ones in the tensor, without sacrificing on the quality of the results; (2) side-information in form of binary pairwise relationships (e.g., an adjacency network) between objects in any tensor mode can also be leveraged, which can be especially useful in ``cold-start'' settings; and (3) the model admits simple inference via batch, as well as online MCMC; the latter allows us to scale up even for dense binary data (i.e., when the number of ones in the tensor/network is also massive). In addition, non-negative factor matrices in our model provide easy interpretability, and the tensor rank is inferred from data. We apply our model on several real-world massive binary tensors, and on massive binary tensors with binary mode-network(s) as side-information.
Changwei Hu, Duke University; Piyush Rai, Duke University; Lawrence Carin, Duke University
ID: 321 (pdf)
Minimizing Expected Losses in Perturbation Models with Multidimensional Parametric Min-cuts +
We consider the problem of learning perturbation-based probabilistic models by computing and differentiating expected losses. This is a challenging computational problem that has traditionally been tackled using Monte Carlo-based methods. In this work, we show how a generalization of parametric min-cuts can be used to address the same problem, achieving high accuracy of faster than a sampling-based baseline. Utilizing our proposed \textit{Skeleton Method}, we show that we can learn the perturbation model so as to directly minimize expected losses. Experimental results show that this approach offers promise as a new way of training structured prediction models under complex loss functions.
Adrian Kim, Seoul National University; Kyomin Jung, Daniel Tarlow, Pushmeet Kohli, Microsoft Research
ID: 326 (pdf)
Learning and Inference in Tractable Probabilistic Knowledge Bases +
Building efficient large-scale knowledge bases (KBs) is a longstanding goal of AI. KBs need to be first-order to be sufficiently expressive, and probabilistic to handle uncertainty, but these lead to intractable inference. Recently, tractable Markov logic (TML) was proposed as the first non-trivial tractable first-order probabilistic representation. This paper describes the first inference and learning algorithms for TML, and its first application to real-world problems. Inference time per query is sublinear in the size of the KB, and supports very large KBs via a disk-based implementation using a relational database engine, and parallelization. Query answering is fast enough for interactive and real-time use. We show that, despite the data being non-i.i.d. in general, maximum likelihood parameters for TML knowledge bases can be computed in closed form. We use our algorithms to build a very large tractable probabilistic KB from numerous heterogeneous data sets. The KB includes millions of objects and billions of parameters. Our experiments show that the learned KB is competitive with existing approaches on challenging tasks in information extraction and integration.
Mathias Niepert, University of Washington; Pedro Domingos, University of Washington
ID: 329 (pdf)
Polynomial-time algorithm for learning optimal tree-augmented dynamic Bayesian networks +
The identification of conditional dependences in longitudinal data is provided through structure learning of dynamic Bayesian networks (DBN). Several methods for DBN learning are concerned with identifying inter-slice dependences, but often disregard the intra-slice connectivity. We propose an algorithm that jointly finds the optimal inter and intra time-slice connectivity in a transition network. The search space is constrained to a class of networks designated by tree–augmented DBN, leading to polynomial time complexity. We assess the effectiveness of the algorithm on simulated data and compare the results to those obtained by a state of the art DBN learning implementation, showing that the proposed algorithm performs very well throughout the different experiments. Further experimental validation is made on real data, by identify- ing non-stationary gene regulatory networks of Drosophila melanogaster.
Alexandra Carvalho, Instituto de Telecomunicações; José Monteiro, IST; Susana Vinga, IDMEC

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