Blocked inference in Bayesian tree substitution grammars

  • Authors:
  • Trevor Cohn;Phil Blunsom

  • Affiliations:
  • University of Sheffield;University of Oxford

  • Venue:
  • ACLShort '10 Proceedings of the ACL 2010 Conference Short Papers
  • Year:
  • 2010

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Abstract

Learning a tree substitution grammar is very challenging due to derivational ambiguity. Our recent approach used a Bayesian non-parametric model to induce good derivations from treebanked input (Cohn et al., 2009), biasing towards small grammars composed of small generalisable productions. In this paper we present a novel training method for the model using a blocked Metropolis-Hastings sampler in place of the previous method's local Gibbs sampler. The blocked sampler makes considerably larger moves than the local sampler and consequently converges in less time. A core component of the algorithm is a grammar transformation which represents an infinite tree substitution grammar in a finite context free grammar. This enables efficient blocked inference for training and also improves the parsing algorithm. Both algorithms are shown to improve parsing accuracy.