Robust Processing of Real-World Natural-Language Texts

Robust Processing of Real-World Natural-Language Texts Jerry R. Hobbs, Douglas E. Appelt, John Bear, and Mabry Tyson Artificial Intelligence Center SR, I International Abstract I1. is often assumed that when natural language processing meets the real world, the ideal of aiming for complete and correct interpretations has to be abandoned. However, our experience with TACITUS; especially in the MUC-3 eval- uation, has shown that. principled techniques fox' syntactic and pragmatic analysis can be bolstered with methods for achieving robust- ness. We describe three techniques for mak- ing syntactic analysis more robust--an agenda- based scheduling parser, a recovery technique for failed parses, and a new technique called ter- minal substring parsing. For pragmatics pro- cessing, we describe how the method of ab- ductive 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 techlfiques have been evaluated and the results of the evaluations are presented. 1 Introduction If automatic text processing is to be a useful enterprise, it. must be demonstrated that the completeness and ac- curacy of the information extracted is adequate for the application one has in nfind. While it is clear that cer- tain applications require only a minimal level of com- petence from a system, it is also true that many appli- cationsrequire a very high degree of completeness and a.ccuracy in text processing, and an increase in capability in either area is a clear advantage. Therefore we adopt an extremely lfigh standard against which the perfor- mance of a text processing system should be measured: it. should recover all information that is implicitly or ex- plicitly present in the text, and it should do so without making mistakes. Tiffs standard is far beyond the state of the art. It is an impossibly high standard for human beings, let alone machines. However, progress toward adequate text pro- cessing is best. served by setting ambitious goals. For this reason we believe that, while it may be necessary in the intermediate term to settle for results that are far short of this ultimate goal, any linguistic theory or system architecture that is adopted should not be demonstra- bly inconsistent with attaining this objective. However, if one is interested, as we are, in the potentially suc- cessful application of these intermediate-term systems to real problems, it is impossible to ignore the question of whether they can be made efficient enough and robust enough for application. 1.1 The TACITUS System The TACITUS text processing system has been under development at SRI International for the last six years. This system has been designed as a first step toward the realization of a system with very high completeness and accuracy in its ability to extract information from text. The general philosophy underlying the design oI this system is that the system, to the maximum extent possible, should not discard any information that might be semantically or pragmatically relevant to a full, cor- rect interpretation. The effect …

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