A Portable Approach to Last Resort Parsing and Interpretation

A PORTABLE APPROACH TO LAST RESORT PARSING AND INTERPRETATION Marcia C. Linebarger, Lewis M. Norton, Deborah A. DaM ABSTRACT This paper describes an approach to robust processing which is domain-independent in its design, yet which can easily take advantage of domain-specific information. Robust processing is well-integrated into standard processing in this approach, requiring essentially only a single new BNF rule in the gram- mar. We describe the results of implementing this approach in two different domains. 1. Introduction For best performance, natural language processing sys- tems must be able to extract as much information as possible from their inputs, even inputs which cannot be fully processed. In order to do this, systems must be equipped with robust processing mechanisms. In addi- tion, cases also occur in which the system has the ability to process an input, given sufficient time, but it is not de- sirable to allow unlimited amounts of processing time. In this paper we describe an approach to robust processing which is domain-independent in its general architecture, but which can be easily customized to particular domains by simply listing key words and/or key concepts. The approach uses the extensive grammar already available to the system for standard processing but augments it with a special BNF rule, called "backup", which is able to prune the wordstream while it searches for key con- cepts. Backup can be triggered either by a failure of nor- mal parsing or by timing out. This approach has been implemented in two distinct domains. In one of these domains, when sufficient time is allotted to attain maxi- mal performance, backup results in an 18% improvement in score. We describe the general approach, discuss how differences in the data in each domain lead to slightly different implementations, and discuss our results. 2. Approach The approach to robust processing which is described in this paper is implemented in the PUNDIT natural language processing system developed at Paramax Sys- tems Corporation [6, 1]. PUNDIT includes a domain- independent, top-down parser [7] which is the primary component involved in robust processing. The key fea- ture of robust processing in PUNDIT is that the parser Paramax Systems Corporation (a Unisys Company) 70 East Swedesford Road Paoli, PA 19301 is allowed to skip over words when it is unable to find a parse using every word. Skipping is an appropriate strategy for the data in the two domains we are working with, because parsing failures tend to be due to extra- neous material such as interpolated irrelevant comments and false starts. Another possible strategy, relaxation of constraints as suggested by [19], is less appropriate for the data we have examined, since few parsing failures are due to violation of grammatical constraints. Skip- ping over words has also been implemented in the ro- bust parsing strategies of Seneff [15] and Strzalkowski [18]; our approach differs from these in that in addition to skipping, it provides a simple way of taking domain- specific knowledge into account in the skipping process. That is, when …

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