Methods for Inference in Graphical Models

• Main Author: Weller, Adrian
• Language: English
• Published: 2014
• Subjects: Computer science
• Online Access: https://doi.org/10.7916/D8JD4VDC

Graphical models provide a flexible, powerful and compact way to model relationships between random variables, and have been applied with great success in many domains. Combining prior beliefs with observed evidence to form a prediction is called inference. Problems of great interest include finding a configuration with highest probability (MAP inference) or solving for the distribution over a subset of variables (marginal inference). Further, these methods are often critical subroutines for learning the relationships. However, inference is computationally intractable in general. Hence, much effort has focused on two themes: finding subdomains where exact inference is solvable efficiently, or identifying approximate methods that work well. We explore both these themes, restricting attention to undirected graphical models with discrete variables. First we address exact MAP inference by advancing the recent method of reducing the problem to finding a maximum weight stable set (MWSS) on a derived graph, which, if perfect, admits polynomial time inference. We derive new results for this approach, including a general decomposition theorem for models of any order and number of labels, extensions of results for binary pairwise models with submodular cost functions to higher order, and a characterization of which binary pairwise models can be efficiently solved with this method. This clarifies the power of the approach on this class of models, improves our toolbox and provides insight into the range of tractable models. Next we consider methods of approximate inference, with particular emphasis on the Bethe approximation, which is in widespread use and has proved remarkably effective, yet is still far from being completely understood. We derive new formulations and properties of the derivatives of the Bethe free energy, then use these to establish an algorithm to compute log of the optimum Bethe partition function to arbitrary epsilon-accuracy. Further, if the model is attractive, we demonstrate a fully polynomial-time approximation scheme (FPTAS), which is an important theoretical result, and demonstrate its practical applications. Next we explore ways to tease apart the two aspects of the Bethe approximation, i.e. the polytope relaxation and the entropy approximation. We derive analytic results, show how optimization may be explored over various polytopes in practice, even for large models, and remark on the observed performance compared to the true distribution and the tree-reweighted (TRW) approximation. This reveals important novel observations and helps guide inference in practice. Finally, we present results related to clamping a selection of variables in a model. We derive novel lower bounds on an array of approximate partition functions based only on the model's topology. Further, we show that in an attractive binary pairwise model, clamping any variable and summing over the approximate sub-partition functions can only increase (hence improve) the Bethe approximation, then use this to provide a new, short proof that the Bethe partition function lower bounds the true value for this class of models. The bulk of this work focuses on the class of binary, pairwise models, but several results apply more generally.