Coordinate Structure Analysis with Global Structural Constraints and Alignment-Based Local Features

Proceedings of the 47th Annual Meeting of the ACL and the 4th IJCNLP of the AFNLP, pages 967–975, Suntec, Singapore, 2-7 August 2009. c⃝2009 ACL and AFNLP Coordinate Structure Analysis with Global Structural Constraints and Alignment-Based Local Features Kazuo Hara Masashi Shimbo Hideharu Okuma Graduate School of Information ScienceYuji Matsumoto Nara Institute of Science and Technology Ikoma, Nara 630-0192, Japan {kazuo-h,shimbo,hideharu-o,matsu}@is.naist.jp Abstract We propose a hybrid approach to coor- dinate structure analysis that combines a simple grammar to ensure consistent global structure of coordinations in a sen- tence, and features based on sequence alignment to capture local symmetry of conjuncts. The weight of the alignment- based features, which in turn determines the score of coordinate structures, is op- timized by perceptron training on a given corpus. A bottom-up chart parsing al- gorithm efficiently finds the best scor- ing structure, taking both nested or non- overlapping flat coordinations into ac- count. We demonstrate that our approach outperforms existing parsers in coordina- tion scope detection on the Genia corpus. 1 Introduction Coordinate structures are common in life sci- ence literature. In Genia Treebank Beta (Kim et al., 2003), the number of coordinate structures is nearly equal to that of sentences. In clinical pa- pers, the outcome of clinical trials is typically de- scribed with coordination, as in Median times to progression and median survival times were 6.1 months and 8.9 months in arm A and 7.2 months and 9.5 months in arm B. (Schuette et al., 2006) Despite the frequency and implied importance of coordinate structures, coordination disambigua- tion remains a difficult problem even for state-of- the-art parsers. Figure 1(a) shows the coordinate structure extracted from the output of Charniak and Johnson’s (2005) parser on the above exam- ple. This is somewhat surprising, given that the symmetry of conjuncts in the sentence is obvious to human eyes, and its correct coordinate structure shown in Figure 1(b) can be readily observed. 6.1 months and 8.9 months in arm A 7.2 months and 9.5 months inarm B and 6.1 months and 8.9 months in arm A 7.2 months and 9.5 months in arm B and (b) (a) Figure 1: (a) Output from the Charniak-Johnson parser and (b) the correct coordinate structure. Structural and semantic symmetry of conjuncts is one of the frequently observed features of coor- dination. This feature has been explored by previ- ous studies on coordination, but these studies often dealt with a restricted form of coordination with apparently too much information provided from outside. Sometimes it was assumed that the co- ordinate structure contained two conjuncts each solely composed of a few nouns; and in many cases, the longest span of coordination (e.g., outer noun phrase scopes) was given a priori. Such rich information might be given by parsers, but this is still an unfounded assumption. In this paper, we approach coordination by tak- ing an extreme stance, and assume that the input is a whole sentence with no subsidiary information except for the parts-of-speech of words. As it assumes minimal …

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