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David Smith Colloquium

David Smith
UMass Amherst Computer Science

Linguistic Inference by Loopy Belief Propagation

Friday, May 15, 3:30 pm, in South College 301

Abstract

Much recent work in natural language processing treats linguistic analysis as an inference problem over graphs. This development opens up useful connections between graph theory, machine learning, and linguistics. We formulate dependency parsing as a graphical model with the novel ingredient of global constraints. We show how to apply loopy belief propagation (BP), a simple and effective tool for approximate learning and inference, to the problem of syntactic analysis. As a parsing algorithm, BP is both asymptotically and empirically efficient. Even with second-order features or latent variables, which would make exact parsing considerably slower or NP-hard, BP needs only O(n^3) time with a small constant factor. Furthermore, such features significantly improve parse accuracy over exact first-order methods. Incorporating additional features increases the runtime additively rather than multiplicatively. Global constraints are propagated by combinatorial optimization algorithms, which greatly improve on collections of local constraints. Finally, I will discuss two extensions to the basic BP parser: a joint model of morphology, syntax, and simple semantic roles and a joint model of sentences and their translations.