tailieunhanh - Báo cáo khoa học: "A Practical Comparison of Parsing Strategies"

Yet important practical considerations do arise when writing programs to understand one aspect or another of natural language utterances. Where, for example, a theorist w i l l characterize a parsing strategy according to its space and/or time requirements in attempting to analyze the worst possible input acc3rding to ~n a r b i t r a r y grammar s t r i c t l y limited in expressive power, the researcher studying Natural Language Processing can be j u s t i f i e d in concerning himself more with issues of practical performance in. | A Practical Comparison of Parsing Strategies Jonathan Slocum Siemens Corporation INTRODUCTION Although the literature dealing with formal and natural languages abounds with theoretical arguments of worstcase performance by various parsing strategies . Griffiths Petrick 1965 Aho Ullman 1972 Graham Harrison Ruzzo 1980 there is little discussion of comparative performance based on actual practice in understanding natural language. Yet important practical considerations do arise when writing programs to understand one aspect or another of natural language utterances Where for example a theorist will characterize a parsing strategy according to its space and or time requirements in attempting to analyze the worst possible input according to an arbitrary grammar strictly limited in expressive power the researcher studying Natural Language Processing can be justified in concerning himself more with issues of practical performance in parsing sentences encountered in language as humans actually use it using a grammar expressed in a form cocvenie to the human linguist who is writing it. Moreover iry occasional poor performance may be quite acceptable particularly if real-time considerations are not involved . if a human querant is not waiting for the answer to his question provided the overall average performance is superior. One example of such a situation Is off-line Machine Translation. This paper has two purposes. One is to report an evaluation of the performance of several parsing strategies in a real-world setting pointing out practical problems in making the attempt Indicating which of the strategies is superior to the others in which situations and most of all determining the reasons why the best strategy outclasses its competition In order to stimulate and direct the design of improvements. The other more important purpose Is to assist in establishing such evaluation as a meaningful and valuable enterprise that contributes to the evolution of Natural Language