Using Parsed Corpora for Structural Disambiguation in the TRAINS Domain

Using Parsed Corpora for Structural Disambiguation in the TRAINS Domain Mark Core Department of Computer Science University of Rochester Rochester, New York 14627 mcore@cs.rochester.edu Abstract This paper describes a prototype disam- biguation module, KANKEI, which was tested on two corpora of the TRAINS project. In ambiguous verb phrases of form V ... NP PP or V ... NP adverb(s), the two corpora have very different PP and adverb attachment patterns; in the first, the cor- rect attachment is to the VP 88.7% of the time, while in the second, the correct at- tachment is to the NP 73.5% of the time. KANKEI uses various n-gram patterns of the phrase heads around these ambiguities, and assigns parse trees (with these ambigu- ities) a score based on a linear combination of the frequencies with which these pat- terns appear with NP and VP attachments in the TRAINS corpora. Unlike previ- ous statistical disambiguation systems, this technique thus combines evidence from bi- grams, trigrams, and the 4-gram around an ambiguous attachment. In the current ex- periments, equal weights are used for sim- plicity but results are still good on the TRAINS corpora (92.2% and 92.4% accu- racy). Despite the large statistical differ- ences in attachment preferences in the two corpora, training on the first corpus and testing on the second gives an accuracy of 90.9%. 1 Introduction The goal of the TRAINS project is to build a com- puterized planning assistant that can interact con- versationally with its user. The current version of this planning assistant, TRAINS 95, is described in (Allen et al., 1995); it passes speech input onto a parser whose chart is used by the dialog manager and other higher-level reasoning components. The planning problems handled involve moving several trains from given starting locations to specified des- tinations on a map display (showing a network of rail lines in the eastern United States). The 95 dialogs are a corpus of people's utterances to the TRAINS 95 system; they contain 773 instances of PP or ad- verb postmodifiers that can attach to either NPs or VPs. Many of these cases were unambiguous, as there was no NP following the VP, or the NP did not follow a verb. Only 275 utterances contained ambiguous constructions and in 73.5% of these, the correct PP/adverb attachment was to the NP. One goal of the TRAINS project is to enhance the TRAINS 95 system sufficiently to handle the more complex TRAINS 91-93 dialogs. This corpus was created between 1991 and 1993 from discussions between humans on transportation problems involv- ing trains. The dialogs deal with time constraints and the added complexity of using engines to pick up boxcars and commodities to accomplish deliv- ery goals. This corpus contains 3201 instances of PP or adverb postmodifiers that can attach to ei- ther NPs or VPs. 1573 of these examples contained both an NP and a VP to which the postmodifier could attach. The postmodifier attached to the VP in 88.7% of these examples. On average, a …

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