A Two-Way Approach to Structural Transfer in MT

A TWO-WAY APPROACH TO STRUCTURAL TRANSFER IN MT Rebecca Root Linguistic.s Research Center University of Texas P.O. Box 7247 Austin, Texas 78712 USA Abstract The METAL machine translation project incorporates two methods of structural transfer - direct transfer and transfer by grammar. In this paper I discuss the strengths and weaknesses of these two approaches in general and with respect to the METAL project, and argue that, for many applications, a combination of the two is preferable to either alone. 1. Introduction One of the central design questions in machine translation is that of the best method of structural transfer, that is, the conversion from the syntactic analysis structure of the source language to the syntactic generation structure of the target language. Although several of the various approaches to this - interlingua, transfer grammar, and direct transfer [Slocum, 84] - share a number of properties which reader a choice among them of relatively little consequence, there is at least one point of variance that can have significant practical ramifications. This is the choice between the ~ of an independent grammar, as one finds in the interlingua and tran~er grammar approaches, and direct transfer, where transfer specifications are tied directly to source language structures. Since each method has its advantages and disadvantages, there is no b~-qi~ for favoring one over the other, independent of a particular application. However, it is highly likely that for a system with any significant range of application, neither approach will be completely satisfactory. Furthermore, decisions made in the design of other components of the system may render a homogeneous approach to transfer impractical. For both of these reasons, we have implemented in METAL a scheme for transfer which is sufficiently flexible to allow for the use of both direct transfer and transfer by grammar. This is done in such a way as to put control of the interaction in the hands of the grammar writer, allowing him to take maximum advantage of the strengths of each approach. in the following, I will contrast the strengths and weaknesses of the two methods mentioned above and illustrate how a combination can inherit the advantages of each by discu~ing our experiences with a combined system in METAL. For the sake of clarity, I will first give an overview of the METAL architecture. 2. Overview of METAL METAL is a machine translation system designed for the translation of technical texts. Currently, it is implemented for German to English translation, but preliminary work has begun on other language pairs. These efforts indicate that, by and large, the design is suitable for application to multiple source and target languages, and work is in progress to make this completely so. Translation proceeds in three phases: analysis, integration, and transfer. The analysis phase consists of parsing the input sentence and building a phrase structure tree annotated with various grammatical features. Anaphoric links are resolved during the integration phase [Weir, 1985]. During the transfer phase, the parse tree is structurally and lexicslly modified according to …

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