Expressing Rhetorical Relations in Instructional Text: A Case Study of the Purposes Relation

Natural language provides an extensive set of lexical and grammatical forms for expressing concepts, a set from which writers choose the particular form that they feel will produce the most effective expression given the communicative context. An important task of the text generation researcher is to specify both the range of these forms and the contexts in which they are used. This paper addresses this issue in the context of the expression of procedural relations between actions in instructional text. It employs the following four-step approach to achieve this goal: (1) collect a corpus of the relevant text type; (2) perform a detailed linguistic study of a portion of this corpus, called the training set, and reserve the remainder as a testing set; (3) implement the results of this study in a text generation system; and (4) compare the output of the system with the text found in the entire corpus. This has resulted in the construction of IMAGENE, an instructional text generation system that embodies a model of the forms of expression consistently used by instructional text writers over a broad range of instruction types. The details of IMAGENE'S treatment of purpose expressions are given as representative of the coverage and form of the full system.