MAP Inference in Chains using Column Generation

Linear chains and trees are basic building blocks in many applications of graphical models. Although exact inference in these models can be performed by dynamic programming, this computation can still be prohibitively expensive with non-trivial target variable domain sizes due to the quadratic dependence on this size. Standard message-passing algorithms for these problems are inefficient because they compute scores on hypotheses for which there is strong negative local evidence. For this reason there has been significant previous interest in beam search and its variants; however, these methods provide only approximate inference. This paper presents new efficient exact inference algorithms based on the combination of it column generation and pre-computed bounds on the model's cost structure. Improving worst-case performance is impossible. However, our method substantially speeds real-world, typical-case inference in chains and trees. Experiments show our method to be twice as fast as exact Viterbi for Wall Street Journal part-of-speech tagging and over thirteen times faster for a joint part-of-speed and named-entity-recognition task. Our algorithm is also extendable to new techniques for approximate inference, to faster two-best inference, and new opportunities for connections between inference and learning.