Full Text Parsing using Cascades of Rules: an Information Extraction Perspective

Proceedings of EACL '99 Full Text Parsing using Cascades of Rules: an Information Extraction Perspective Fabio Ciravegna and Alberto Lavelli ITC-irst Centro per la Ricerca Scientifica e Tecnologica via Sommarive, 18 38050 Povo (TN) ITALY {cirave[lavelli} Qirst.itc.it Abstract This paper proposes an approach to full parsing suitable for Information Extrac- tion from texts. Sequences of cascades of rules deterministically analyze the text, building unambiguous structures. Ini- tially basic chunks are analyzed; then ar- gumental relations are recognized; finally modifier attachment is performed and the global parse tree is built. The ap- proach was proven to work for three lan- guages and different domains. It was im- plemented in the IE module of FACILE, a EU project for multilingual text classi- fication and !E. 1 Introduction Most successful approaches in IE (Appelt et al., 1993; Grishman, 1995; Aone et al., 1998) make a very poor use of syntactic information. They are generally based on shallow parsing for the anal- ysis of (non recursive) NPs and Verba~ Groups (VGs). After such step regular patterns are ap- plied in order to trigger primitive actions that fill template(s); meta-rules are applied to patterns to cope with different syntactic clausal forms (e.g., passive forms). If we consider the most com- plex MUC-7 task (i.e., the Scenario Template task (MUC7, 1998)), the current technology is not able to provide results near an operational level (ex- pected F(1)=75%; the best system scored about 50% (Aone et al., 1998)). One of the limita- tions of the current technology is the inability to extract (and to represent) syntactic relations among elements in the sentence, i.e. grammati- cal functions and thematic roles. Scenario Tem- plate recognition needs the correct treatment of syntactic relations at both sentence and text level (Aone et al., 1998). Full parsing systems are gen- erally able to correctly model syntactic relations, but they tend to be slow (because of huge search spaces) and brittle (because of gaps in the gram- mar). The use of big grammars partially solves the problem of gaps but worsens the problem of huge search spaces and makes grammar modifications difficult (Grishman, 1995). Grammar modifica- tions are always to be taken into account. Many domain-specific texts present idiosyncratic phe- nomena that require non-standard rules. Often such phenomena are limited to some cases only (e.g., some types of coordinations are applied to people only and not to organizations). Inserting generic rules for such structures introduces (use- less) extra complexity into the search space and - when applied indiscriminately (e.g., on classes other than people) - can worsen the system re- sults. It is not clear how semantic restrictions can be introduced into (big) generic grammars. In this paper we propose an approach to full parsing for IE based on cascades of rules. The approach is inspired by the use of finite-state cas- cades for parsing (e.g., (Abney, 1996) uses them in a project for inducing lexical dependencies from corpora). Our work is interesting in an IE per- spective because it proposes: • a …

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