Coping With Ambiguity in a Large-Scale Machine Translation System

In an interlingual knowledge-based machine translation system, ambiguity arises when the source 1.qnguage analyzer produces more than one interlingua expression for a source sentence. This can have a negative impact on translation quality, since a target sentence may be produced from an unintended meaning. In this paper we describe the ,nethods nsed in the KANT machine translation system to reduce or eliminate ambiguity in a large-scale application domain. We also test these methods on a large corpus of test sentences, in order to illustrate how the different disambiguation methods redtuce the average number of parses per sentence, 1 I n t r o d u c t i o n The KANT system [Mitamura etal . , 1991] is a system for Knowledge-basexl, Accurate Natural-language Translation. The system is used in focused technical domains for multilingual translation of controlled source language documents. KANT is an interlingua-based system: the sonrce language analyzer produces an interlingua expression for each source sentence, and this interlingua is processed to produce the corresponding target sentence. The problen3 el' ambiguity arises when the system produces more that~ ()tie interlingua representation for a single input sentence. If the goal is to automate translation and produce output that does not require post-editing, then the presence of ambiguity has a negative impact on translation quality, since a target sentence may he produced from an unintended meaning. When it is possible to limit tile interpretations of a sentence to just those that are coherent in the translation domain, then the accuracy of the MT system is enhanced. Ambiguity can occnr at different levels of processing in source analysis. In this paper, we describe how we cope with ambiguity in the KANT controlled lexicon, grammar, and semantic domain model, and how these :ire designed to reduce or eliminate ambiguity in a given translation domain. 2 C o n s t r a i n i n g t h e S o u r c e T e x t The KANT domain lexicon and grammar are a constrained subset of the general source language lexicon and gra,nmar. The strategy of constraining the source text has three main I