Bilingual-Dictionary Adaptation to Domains

Bilingual-Dictionary Adaptation to Domains Hiroyuki Kaji Central Research Laboratory, Hitachi, Ltd. 1-280 Higashi-Koigakubo, Kokubunji-shi, Tokyo 185-8601, Japan kaji@crl.hitachi.co.jp Abstract Two methods using comparable corpora to se- lect translation equivalents appropriate to a do- main were devised and evaluated. The first method ranks translation equivalents of a target word according to similarity of their contexts to that of the target word. The second method ranks translation equivalents according to the ratio of associated words that suggest them. An experiment using the EDR bilingual dictionary together with Wall Street Journal and Nihon Keizai Shimbun corpora proved that the method using the ratio of associated words outperforms the method based on contextual similarity. Namely, in a quantitative evaluation using pseudo words, the maximum F-measure of the former method was 86%, while that of the latter method was 82%. 1 Introduction It is well known that appropriate translations for a word vary with domains, and bilingual-dictionary adaptation to domains is an effective way to improve the performance of, for example, machine translation and cross-language information retrieval. However, bilingual dictionaries have commonly been adapted to domains on the basis of lexicographers’ intuition. It is thus desirable to develop an automated method for bilingual-dictionary adaptation. Technologies for extracting pairs of translation equivalents from parallel corpora have been estab- lished (Gale and Church 1991; Dagan, et al. 1993; Fung 1995; Kitamura and Matsumoto 1996; Melamed 1997). They can, naturally, be used to adapt a bilingual dictionary to domains, that is, to select corpus-relevant translation equivalents from among those provided by an existing bilingual dic- tionary. However, their applicability is limited be- cause of the limited availability of large parallel corpora. Methods of bilingual-dictionary adaptation using weakly comparable corpora, i.e., a pair of two language corpora of the same domain, are therefore required. There are a number of previous works related to bilingual-dictionary adaptation using comparable corpora. Tanaka and Iwasaki’s (1996) optimization method for a translation-probability matrix mainly aims at adapting a bilingual dictionary to domains. However, it is hampered by a huge amount of com- putation, and was only demonstrated in a small-scale experiment. Several researchers have developed a contextual-similarity-based method for extracting pairs of translation equivalents (Kaji and Aizono 1996; Fung and McKeown 1997; Fung and Yee 1998; Rapp 1999). It is computationally efficient compared to Tanaka and Iwasaki’s method, but the precision of extracted translation equivalents is still not acceptable. In the light of these works, the author proposes two methods for bilingual-dictionary adaptation. The first one is a variant of the contex- tual-similarity-based method for extracting pairs of translation equivalents; it focuses on selecting cor- pus-relevant translation equivalents from among those provided by a bilingual dictionary. This select- ing may be easier than finding new pairs of transla- tion equivalents. The second one is a newly devised method using the ratio of associated words that sug- gest each translation equivalent; it was inspired by a research on word-sense disambiguation using bilin- gual comparable corpora (Kaji and Morimoto 2002). The two methods were evaluated and …

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