The Tacitus System: The Muc-3 Experience

Abstract

It is often assumed that when natural language processing meets the real world, the idea of aiming for complete and correct interpretations has to be abandoned. However, our experience with TACITUS, especially in the MUC-3 evaluation, has shown that principled techniques for syntactic and pragmatic analysis can be bolstered with methods for achieving robustness. We describe and evaluate a method for dealing with unknown words and a method for filtering out sentences irrelevant to the task. We describe three techniques for making syntactic analysis more robust – an agenda-based scheduling parser, a recovery technique for failed parses, and a new technique called terminal sub-string parsing. For pragmatics processing, we describe how the method of abductive inference is inherently robust, in that an interpretation is always possible, so that in the absence of the required world knowledge, performance degrades gracefully. Each of these techniques has been evaluated, and the results of the evaluations are presented.