Fertilization of Case Frame Dictionary for Robust Japanese Case Analysis

Fertilization of Case Frame Dictionary for Robust Japanese Case Analysis Daisuke Kawahara† and Sadao Kurohashi†‡ †Graduate School of Information Science and Technology, University of Tokyo ‡PRESTO, Japan Science and Technology Corporation (JST) {kawahara,kuro}@kc.t.u-tokyo.ac.jp Abstract This paper proposes a method of fertilizing a Japanese case frame dictionary to handle com- plicated expressions: double nominative sen- tences, non-gapping relation of relative clauses, and case change. Our method is divided into two stages. In the first stage, we parse a large corpus and construct a Japanese case frame dic- tionary automatically from the parse results. In the second stage, we apply case analysis to the large corpus utilizing the constructed case frame dictionary, and upgrade the case frame dictio- nary by incorporating newly acquired informa- tion. 1 Introduction To understand a text, it is necessary to find out relations between words in the text. What is required to do so is a case frame dictionary. It describes what kinds of cases each verb has and what kinds of nouns can fill a case slot. Since these relations have millions of combinations, it is difficult to construct a case frame dictio- nary by hand. We proposed a method to con- struct a Japanese case frame dictionary auto- matically by arranging large volumes of parse results by coupling a verb and its closest case component (Kawahara and Kurohashi, 2001). This case frame dictionary, however, could not handle complicated expressions: double nomi- native sentences, non-gapping relation of rela- tive clauses, and case change. This paper proposes a method of fertiliz- ing the case frame dictionary to handle these complicated expressions. We take an iterative method which consists of two stages. This means gradual learning of what is understood by an analyzer in each stage. In the first stage, we parse a large raw corpus and construct a Japanese case frame dictionary automatically from the parse results. This is the method pro- posed by (Kawahara and Kurohashi, 2001). In the second stage, we apply case analysis to the large corpus utilizing the constructed case frame dictionary, and upgrade the case frame dictio- nary by incorporating newly acquired informa- tion. We conducted a case analysis experiment with the upgraded case frame dictionary, and its evaluation showed effectiveness of the fertil- ization process. 2 Japanese Grammar We introduce Japanese grammar briefly in this section. Japanese is a head-final language. Word or- der does not play a case-marking role. Instead, postpositions function as case markers (CMs). The basic structure of a Japanese sentence is as follows: (1) kare he ga nom-CM hon book wo acc-CM kaku write (he writes a book) ga and wo are postpositions which mean nom- inative and accusative, respectively. kare ga and hon wo are case components, and kaku is a verb1. There are two phenomena that case markers are hidden. A modifying clause is left to the modified noun in Japanese. In this paper, we call a noun modified by a clause clausal modifiee. A clausal modifiee is usually a case component for the verb …

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