Lexicalized Grammar Acquisition

Lexicalized Grammar Acquisition Yusuke Miyaot� Takashi Ninomiyatt�Jun'ichi Tsujiit f Department of Computer Science, University of Tokyo Bongo 7-3-1, Bunkyo-ku, Tokyo 113-0033 JAPAN $CREST, JST (Japan Science and Technology Corporation) Honcho 4-1-8, Kawaguchi-shi, Saitama 332-0012 JAPAN {yusuke, ninomi , tsuj ii}@is s .u-tokyo ac jp Abstract This paper presents a formalization of automatic grammar acquisition that is based on lexicalized grammar for- malisms (e.g. LTAG and HPSG). We state the conditions for the consistent ac- quisition of a unique lexicalized gram- mar from an annotated corpus. 1 Introduction Linguistically motivated and computationally ori- ented grammar theories take the form of lexical- ized grammar formalisms; examples include Lex- icalized Tree Adjoining Grammar (LTAG) (Sch- abes et al., 1988), Combinatory Categorial Gram- mar (Steedman, 2000), and Head-driven Phrase Structure Grammar (HPSG) (Sag and Wasow, 1999). They have been a great success in terms of linguistic analysis and efficiency in the parsing of real-world texts. However, such grammars have not generally been considered suitable for the syn- tactic analysis within practical NLP systems be- cause considerable effort is required to develop and maintain lexicalized grammars that are both robust and provide broad coverage. One novel approach to grammar development is based on the automatic acquisition of lexicalized grammars from annotated corpora. Since lexical- ized grammars represent grammatical constraints with a few grammar rules and a large number of lexical entries, the rules are quite easy to write but the construction of a lexicon is unrealistic. The idea in this study is to automatically obtain the lex- ical entries from an annotated corpus, which will greatly reduce the cost of building the grammar. The approach has the following advantages. First, the grammars obtained will be robust be- cause appropriate lexical entries are consistently acquired even for constructions beyond grammar developers' intuition. Secondly, the grammar de- velopers are simply required to annotate the closed set of the training corpus, where various heuris- tic and statistical methods are applicable. Con- sistency between the grammar rules and the ob- tained lexical entries is assured independently of the methods of annotation. Lastly, the validity of the grammar theories is evaluated on real-world texts. A degree of low coverage by a linguistically motivated grammar does not necessarily reflect in- adequacy of the grammar theories; a lack of appro- priate lexical entries may also be responsible. The analysis of obtained grammars gives us grounds for discussing the pros and cons of the theories. The studies on the extraction of LTAG (Xia, 1999; Chen and Vijay-Shanker, 2000; Chiang, 2000) and CCG (Hockenmaier and Steedman, 2002) represent the first attempts at the acquisition of linguistically motivated grammars from anno- tated corpora. Those studies are limited to specific formalisms, and can be interpreted as instances of our approach as described in Section 3. This pa- per does not describe any concrete algorithms for grammar acquisition that depend on specific gram- mar formalisms. The contribution of our work is to formally state the conditions required for the ac- quisition of lexicalized grammars and to demon- …

Content truncated to 500 words