Japanese Case Structure Analysis

Japanese Case Structure Analysis by Unsupervised Construction of a Case Frame Dictionary Daisuke Kawahara, Nobuhiro Kaji and Sadao Kurohashi Graduate School of Informatics, Kyoto University Yoshida-Honmachi, Sakyo-ku, Kyoto, 606–8501, Japan {kawahara,kaji,kuro}@pine.kuee.kyoto-u.ac.jp Abstract In Japanese, case structure analysis is very im- portant to handle several troublesome charac- teristics of Japanese such as scrambling, omis- sion of case components, and disappearance of case markers. However, for lack of a wide- coverage case frame dictionary, it has been dif- ficult to perform case structure analysis accu- rately. Although several methods to construct a case frame dictionary from analyzed corpora have been proposed, they cannot avoid data sparseness problem. This paper proposes an un- supervised method of constructing a case frame dictionary from an enormous raw corpus by us- ing a robust and accurate parser. It also pro- vides a case structure analysis method based on the constructed dictionary. 1 Introduction Syntactic analysis, or parsing has been a main objective in Natural Language Processing. In case of Japanese, however, syntactic analysis cannot clarify relations between words in sen- tences because of several troublesome character- istics of Japanese such as scrambling, omission of case components, and disappearance of case markers. Therefore, in Japanese sentence analy- sis, case structure analysis is an important issue, and a case frame dictionary is necessary for the analysis. Some research institutes have constructed Japanese case frame dictionaries manually (Ike- hara et al., 1997; Information-Technology Pro- motion Agency, Japan, 1987). However, it is quite expensive, or almost impossible to con- struct a wide-coverage case frame dictionary by hand. Others have tried to construct a case frame dictionary automatically from analyzed corpora (Utsuro et al., 1998). However, existing syntac- tically analyzed corpora are too small to learn a dictionary, since case frame information consists of relations between nouns and verbs, which mul- tiplies to millions of combinations. Based on such a consideration, we took the following unsupervised learning strategy to the Japanese case structure analysis: 1. At first, a robust and accurate parser is de- veloped, which does not utilize a case frame dictionary, 2. a very large corpus is parsed by the parser, 3. reliable noun-verb relations are extracted from the parse results, and a case frame dic- tionary is constructed from them, and 4. the dictionary is utilized for case structure analysis. 2 Characteristics of Japanese language and necessity of case structure analysis In Japanese, postpositions function as case markers (CMs) and a verb is final in a sentence. The basic structure of a Japanese sentence is as follows: (1) kare ga coat wo kiru. he nominative-CM coat accusative-CM wear (he wears a coat) A clause modifier is left to the modified noun as follows: (2) kare ga kite-iru coat he nom-CM wear coat (the coat he wears) The modified noun followed by a postposition then becomes a case component of a matrix verb. The typical structure of a Japanese complex sen- tence is as follows: (3) boushi no iro wa kite-iru hat coat niof color topic-marker wear awaseru. coat …

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