Parsing with Generative Models of Predicate-Argument Structure

Parsing with generative models of predicate-argument structure Julia Hockenmaier IRCS, University of Pennsylvania, Philadelphia, USA and Informatics, University of Edinburgh, Edinburgh, UK juliahr@linc.cis.upenn.edu Abstract The model used by the CCG parser of Hockenmaier and Steedman (2002b) would fail to capture the correct bilexical dependencies in a language with freer word order, such as Dutch. This paper argues that probabilistic parsers should therefore model the dependencies in the predicate-argument structure, as in the model of Clark et al. (2002), and defines a generative model for CCG derivations that captures these dependencies, includ- ing bounded and unbounded long-range dependencies. 1 Introduction State-of-the-art statistical parsers for Penn Treebank-style phrase-structure grammars (Collins, 1999), (Charniak, 2000), but also for Categorial Grammar (Hockenmaier and Steedman, 2002b), include models of bilexical dependencies defined in terms of local trees. However, this paper demonstrates that such models would be inadequate for languages with freer word order. We use the example of Dutch ditransitives, but our argument equally applies to other languages such as Czech (see Collins et al. (1999)). We argue that this problem can be avoided if instead the bilexical dependencies in the predicate-argument structure are captured, and propose a generative model for these dependencies. The focus of this paper is on models for Combina- tory Categorial Grammar (CCG, Steedman (2000)). Due to CCG’s transparent syntax-semantics inter- face, the parser has direct and immediate access to the predicate-argument structure, which includes not only local, but also long-range dependencies arising through coordination, extraction and con- trol. These dependencies can be captured by our model in a sound manner, and our experimental re- sults for English demonstrate that their inclusion im- proves parsing performance. However, since the predicate-argument structure itself depends only to a degree on the grammar formalism, it is likely that parsers that are based on other grammar for- malisms could equally benefit from such a model. The conditional model used by the CCG parser of Clark et al. (2002) also captures dependencies in the predicate-argument structure; however, their model is inconsistent. First, we review the dependency model proposed by Hockenmaier and Steedman (2002b). We then use the example of Dutch ditransitives to demon- strate its inadequacy for languages with a freer word order. This leads us to define a new generative model of CCG derivations, which captures word-word de- pendencies in the underlying predicate-argument structure. We show how this model can capture long-range dependencies, and deal with the pres- ence of multiple dependencies that arise through the presence of long-range dependencies. In our current implementation, the probabilities of derivations are computed during parsing, and we discuss the dif- ficulties of integrating the model into a probabilis- tic chart parsing regime. Since there is no CCG treebank for other languages available, experimen- tal results are presented for English, using CCGbank (Hockenmaier and Steedman, 2002a), a translation of the Penn Treebank to CCG. These results demon- strate that this model benefits greatly from the inclu- sion of long-range dependencies. 2 A model of surface dependencies Hockenmaier and Steedman (2002b) …

Content truncated to 500 words