A Fully-Lexicalized Probabilistic Model for Japanese Syntactic and Case Structure Analysis

Proceedings of the Human Language Technology Conference of the North American Chapter of the ACL, pages 176–183, New York, June 2006. c⃝2006 Association for Computational Linguistics A Fully-Lexicalized Probabilistic Model for Japanese Syntactic and Case Structure Analysis Daisuke Kawahara∗and Sadao Kurohashi† Graduate School of Information Science and Technology, University of Tokyo 7-3-1 Hongo Bunkyo-ku, Tokyo, 113-8656, Japan {kawahara,kuro}@kc.t.u-tokyo.ac.jp Abstract We present an integrated probabilistic model for Japanese syntactic and case structure analysis. Syntactic and case structure are simultaneously analyzed based on wide-coverage case frames that are constructed from a huge raw corpus in an unsupervised manner. This model se- lects the syntactic and case structure that has the highest generative probability. We evaluate both syntactic structure and case structure. In particular, the experimen- tal results for syntactic analysis on web sentences show that the proposed model significantly outperforms known syntactic analyzers. 1 Introduction Case structure (predicate-argument structure or log- ical form) represents what arguments are related to a predicate, and forms a basic unit for conveying the meaning of natural language text. Identifying such case structure plays an important role in natural lan- guage understanding. In English, syntactic case structure can be mostly derived from word order. For example, the left ar- gument of the predicate is the subject, and the right argument of the predicate is the object in most cases. Blaheta and Charniak proposed a statistical method ∗Currently, National Institute of Information and Communi- cations Technology, JAPAN, dk@nict.go.jp †Currently, Graduate School of Informatics, Kyoto Univer- sity, kuro@i.kyoto-u.ac.jp for analyzing function tags in Penn Treebank, and achieved a really high accuracy of 95.7% for syn- tactic roles, such as SBJ (subject) and DTV (da- tive) (Blaheta and Charniak, 2000). In recent years, there have been many studies on semantic structure analysis (semantic role labeling) based on PropBank (Kingsbury et al., 2002) and FrameNet (Baker et al., 1998). These studies classify syntactic roles into se- mantic ones such as agent, experiencer and instru- ment. Case structure analysis of Japanese is very differ- ent from that of English. In Japanese, postpositions are used to mark cases. Frequently used postposi- tions are “ga”, “wo” and “ni”, which usually mean nominative, accusative and dative. However, when an argument is followed by the topic-marking post- position “wa”, its case marker is hidden. In addi- tion, case-marking postpositions are often omitted in Japanese. These troublesome characteristics make Japanese case structure analysis very difficult. To address these problems and realize Japanese case structure analysis, wide-coverage case frames are required. For example, let us describe how to apply case structure analysis to the following sen- tence: bentou-wa taberu lunchbox-TM eat (eat lunchbox) In this sentence, taberu (eat) is a verb, and bentou- wa (lunchbox-TM) is a case component (i.e. argu- ment) of taberu. The case marker of “bentou-wa” is hidden by the topic marker (TM) “wa”. The an- alyzer matches “bentou” (lunchbox) with the most 176 suitable case slot (CS) in the following case frame of “taberu” (eat). CS examples taberu ga person, child, boy, · · …

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