tailieunhanh - Báo cáo khoa học: "Prefix Probability for Probabilistic Synchronous Context-Free Grammars"

We present a method for the computation of prefix probabilities for synchronous contextfree grammars. Our framework is fairly general and relies on the combination of a simple, novel grammar transformation and standard techniques to bring grammars into normal forms. | Prefix Probability for Probabilistic Synchronous Context-Free Grammars Mark-Jan Nederhof School of Computer Science University of St Andrews North Haugh St Andrews Fife Ky16 9SX United Kingdom Giorgio Satta Dept. of Information Engineering University of Padua via Gradenigo 6 A I-35131 Padova Italy satta@ Abstract We present a method for the computation of prefix probabilities for synchronous context-free grammars. Our framework is fairly general and relies on the combination of a simple novel grammar transformation and standard techniques to bring grammars into normal forms. 1 Introduction Within the area of statistical machine translation there has been a growing interest in so-called syntaxbased translation models that is models that define mappings between languages through hierarchical sentence structures. Several such statistical models that have been investigated in the literature are based on synchronous rewriting or tree transduction. Probabilistic synchronous context-free grammars PSCFGs are one among the most popular examples of such models. PSCFGs subsume several syntax-based statistical translation models as for instance the stochastic inversion transduction grammars of Wu 1997 the statistical model used by the Hiero system of Chiang 2007 and systems which extract rules from parsed text as in Galley et al. 2004 . Despite the widespread usage of models related to PSCFGs our theoretical understanding of this class is quite limited. In contrast to the closely related class of probabilistic context-free grammars a syntax model for which several interesting mathematical and statistical properties have been investigated as for instance by Chi 1999 many theoretical problems are still unsolved for the class of PSCFGs. 460 This paper considers a parsing problem that is well understood for probabilistic context-free grammars but that has never been investigated in the context of PSCFGs viz. the computation of prefix .