Guessing Parts-of-Speech of Unknown Words Using Global Information

Proceedings of the 21st International Conference on Computational Linguistics and 44th Annual Meeting of the ACL, pages 705–712, Sydney, July 2006. c⃝2006 Association for Computational Linguistics Guessing Parts-of-Speech of Unknown Words Using Global Information Tetsuji Nakagawa Corporate R&D Center Oki Electric Industry Co., Ltd. 2−5−7 Honmachi, Chuo-ku Osaka 541−0053, Japan nakagawa378@oki.com Yuji Matsumoto Graduate School of Information Science Nara Institute of Science and Technology 8916−5 Takayama, Ikoma Nara 630−0101, Japan matsu@is.naist.jp Abstract In this paper, we present a method for guessing POS tags of unknown words us- ing local and global information. Al- though many existing methods use only local information (i.e. limited window size or intra-sentential features), global in- formation (extra-sentential features) pro- vides valuable clues for predicting POS tags of unknown words. We propose a probabilistic model for POS guessing of unknown words using global information as well as local information, and estimate its parameters using Gibbs sampling. We also attempt to apply the model to semi- supervised learning, and conduct experi- ments on multiple corpora. 1 Introduction Part-of-speech (POS) tagging is a fundamental language analysis task. In POS tagging, we fre- quently encounter words that do not exist in train- ing data. Such words are called unknown words. They are usually handled by an exceptional pro- cess in POS tagging, because the tagging sys- tem does not have information about the words. Guessing the POS tags of such unknown words is a difficult task. But it is an important issue both for conducting POS tagging accurately and for creating word dictionaries automatically or semi- automatically. There have been many studies on POS guessing of unknown words (Mori and Na- gao, 1996; Mikheev, 1997; Chen et al., 1997; Na- gata, 1999; Orphanos and Christodoulakis, 1999). In most of these previous works, POS tags of un- known words were predicted using only local in- formation, such as lexical forms and POS tags of surrounding words or word-internal features (e.g. suffixes and character types) of the unknown words. However, this approach has limitations in available information. For example, common nouns and proper nouns are sometimes difficult to distinguish with only the information of a sin- gle occurrence because their syntactic functions are almost identical. In English, proper nouns are capitalized and there is generally little ambi- guity between common nouns and proper nouns. In Chinese and Japanese, no such convention ex- ists and the problem of the ambiguity is serious. However, if an unknown word with the same lex- ical form appears in another part with informa- tive local features (e.g. titles of persons), this will give useful clues for guessing the part-of-speech of the ambiguous one, because unknown words with the same lexical form usually have the same part-of-speech. For another example, there is a part-of-speech named sahen-noun (verbal noun) in Japanese. Verbal nouns behave as common nouns, except that they are used as verbs when they are followed by a verb “suru”; e.g., a verbal noun “dokusho” means “reading” and “dokusho-suru” is a verb meaning to “read books”. …

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