Parsing with Principles and Probabilities

Parsing with Principles and Probabilities Andrew Fordham SCS Research Group Department of Sociology University of Surrey Guildford Surrey, GU2 5XH, UK aj fesoc, surrey, ac. uk Abstract This paper is an attempt to bring together two ap- proaches to language analysis. The possible use of prob- abilistic information in principle-based grammars and parsers is considered, including discussion on some the- oretical and computational problems that arise. Finally a partial implementation of these ideas is presented, along with some preliminary results from testing on a small set of sentences. Introduction Both principle-based parsing and probabilistic methods for the analysis of natural language have become pop- ular in the last decade. While the former borrows from advanced linguistic specifications of syntax, the latter has been more concerned with extracting distributional regularities from language to aid the implementation of NLP systems and the analysis of corpora. These symbolic and statistical approaches axe begin- ning to draw together as it becomes clear that one can- not exist entirely without the other: the knowledge of language posited over the years by theoretical linguists has been useful in constraining and guiding statistical approaches, and the corpora now available to linguists have resurrected the desire to account for real language data in a more principled way than had previously been attempted. This paper falls directly between these approaches, using statistical information derived from corpora anal- ysis to weight syntactic analyses produced by a 'prin- ciples and parameters' parser. The use of probabilistic information in principle-based grammars and parsers is considered, including discussion on some theoretical and computational problems that arise. Finally a pax- tial implementation of these ideas is presented, along with some preliminary results from testing on a small set of sentences. Government.-Binding Theory The principles and paxameters paradigm in linguistics is most fully realised in the Government-Binding Theory (GB) of Chomsky [Chomsky1981, Chomsky19861 and others. The grammar is divided into modules which Matthew Crocker Centre for Cognitive Science University of Edinburgh 2 Buccleuch Place Edinburgh, EH8 9LW Scotland mwc@cogsc±, ed. ac. uk filter out ungrammatical structures at the various levels of representation; these levels are related by general transformations. A sketch of the organisation of GB (the 'T-model') is shown in figure I. D-Structure ( X'-theory, lexicsI insertion, O-criterion) S-Struaure (Case Theory, Subjacency) Pf// k-movement (move.a) Phonetic Form Logical Form (Empty Category Principle, Bindino Theoq) Figure 1: The T-model of grammar Little work has been done on the complexity of al- gorithms used to parse with a principle-based gram- mar, since such grammars do not exist as accepted mathematically well-defined constructs. It has been estimated that in general, principle-based parsing can only be accomplished in exponential time, i.e. 0(2") [Berwick and Weinberg1984, Weinberg19881. A feature of principle-based grammars is their po- tential to assign some meaningful representation to a string which is strictly ungrammatical. It is an inherent feature of phrase structure grammars that they classify the strings of words from a language into two (infinite) sets, one containing the grammat- ical strings and the …

Content truncated to 500 words