Hybrid Disambiguation of Prepositional Phrase Attachment and Interpretation

! Hybrid Disambiguation of Prepositional Phrase Attachment and Interpretation Sven Hartrumpf Applied Computer Science VII (AI) University of Hagen 58084 Hagen, Germany Sven.Hartrumpf@fernuni-hagen.de Abstract In this paper, a hybrid disambiguation method for the prepositional phrase (PP) attachment and interpretation problem is presented. 1 The data needed, semantic PP interpretation rules and an annotated corpus, is described first. Then the three major steps of the disambigua- tion method are: explained. Cross-validated evaluation results', for German (88.6-94.4% cor- rect for binary attachment ambiguities, 83.3- 92.5% correct for interpretation ambiguities) show that disambiguation methods combin- ing interpretation! rules and statistical methods might yield significantly better results than non- hybrid disambiguation methods. 1 Introduction The problem of prepositional phrase (PP) at- tachment ambigu!ty is one of the most famous problems in natural language processing (NLP). In recent years, many statistical solutions have been proposed: lexical associations (see (Hin- dle and Rooth, 1993)); error-driven transfor- mation learning (see (Brill and Resnik, 1994), extensions by (Ye h and Vilain, 1998)); backed- off estimation (see (Collins and Brooks, 1995), extended to the multiple PP attachment prob- lem by (Merlo et al., 1997)); loglinear model (see (Franz, 1996b), (Franz, 1996a, pp. 97- 108)); maximum:entropy model (see (Ratna- parkhi, 1998; Ratnaparkhi et al., 1994)). The disambiguation method in this paper has two key features: First, it tries to solve the 1This disambiguation method was developed for an NLI in the Virtuelle Wissensfabrik ( Virtual Knowledge Factory, see (Knoll et al., 1998)), a project funded by the German state Nordrhein-Westfalen, which supported this research in part. I would like to thank Rainer Oss- wald and the anonymous reviewers for their useful com- ments and suggestions. PP attachment problem and the PP interpre- tation problem. Second, it is hybrid as it com- bines more traditional PP interpretation rules and statistical methods. 2 Data 2.1 PP interpretation rules One central component for the disambigua- tion method presented in this paper are se- mantic interpretation rules for PPs. A PP interpretation rule consists of a premise and a conclusion. The premise of an inter- pretation rule describes under which condi- tions the PP interpretation specified by the rule's conclusion can be valid. Two example rules for the local and contents interpretation of 'fiber' ('about'/'above'/'on'/'over'/'via'/...) are shown in Figure 1. As (at least) five more interpretations of 'fiber' are possible, the ambi- guity degree for the interpretation of such a PP is (at least) seven. The premise of a rule is a set of feature struc- ture constraints (including negated and disjunc- tive constraints and defining an underspecified feature structure) that refer to the following fea- tures of the preposition's sister NP (nominal phrase) and the preposition's mother NP or V (verb). (The features that are only refered to for the sister NP are marked by an S.) case (S) syntactic case: genitive, dative, and accusative for German PPs num (S) syntactic number: singular and plu- ral in German sort a semantic sort value (atomic or dis- junctive value) from a predefined ontol- ogy …

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