Definiteness Predictions for Japanese Noun Phrases

One of the major problems when translating from Japanese into a European language such as German or English is to determine definiteness of noun phrases in order to choose the correct determiner in the target language. Even though in Japanese, noun phrase reference is said to depend in large parts on the discourse context, we show that in many cases there also exist linguistic markers for definiteness. We use these to build a rule hierarchy that predicts 79,5% of the articles with an accuracy of 98,9% from syntactic-semantic properties alone, yielding an efficient pre-processing tool for the computationally expensive context checking. 1 I n t r o d u c t i o n One of the major problems when translating from Japanese into a European language such as German or English is the insertion of articles. Both German and English distinguish between the definite and indefinite article, the former, in general, indicating some degree of familiarity with the referent, the latter referring to something new. Thus by using a definite article, the speaker expects the hearer to be able to identify the object he is talking about, whilst with the use of an indefinite article, a new referent is introduced into the discourse context (Heim, 1982). In contrast, the reference of Japanese noun phrases depends in large parts on the discourse " I would like to thank my colleagues Johan Bos, BjSrn Gambiick, Yoshiki Mori, Michael Paul, Manfred Pinkal, C.J. Rupp, Atsuko Shimada, Kristina Striegnitz and Karsten Worm for their valuable comments and support. This research was supported by the German Ministry of Education, Science, Research and Technology (BMBF) within the Verbmobil framework under grant no. 01 IV 701 R4. context, taking a previous mention of an object and all properties that can be inferred from it, as well as world knowledge as indicators for definite reference. Any noun phrase whose referent cannot be recovered from the discourse context will in turn be taken as indefinite. However, noun phrases can also be explicitly marked for definiteness, forcing an interpretation of the referent independent of the discourse context. In this way, it is possible to trigger accommodation of previously unknown specific referents, or to get an indefinite reading even if an object of the same type has already been introduced. For machine translation, it is important to find a systematic way of extracting the syntactic and semantic information responsible for marking the reference of noun phrases, in order to correctly choose the articles to be used in the target language. For this paper, we propose a rule hierarchy for this purpose, that can be used as a preprocessing tool to context checking. All noun phrases marked for definiteness in any way are assigned their referential property, leaving the others underspecified. After giving a short outline of related work in the next section, we will introduce our rule hierarchy in section 3. The resulting algorithm will be evaluated in section 4, and in section 5 we will address implementational issues. Finally, in section 6 we give a conclusion. 2 R e l a t e d W o r k The problem of article selection when translating from Japanese into any language requiring the use of articles has only been addressed systematically by a few authors. (Murata and Nagao, 1993) define a heuristic rule base for definiteness assignment, consisting of 86 weighted rules. These rules use surface in-