Exploiting Heterogeneous Treebanks for Parsing

Proceedings of the 47th Annual Meeting of the ACL and the 4th IJCNLP of the AFNLP, pages 46–54, Suntec, Singapore, 2-7 August 2009. c⃝2009 ACL and AFNLP Exploiting Heterogeneous Treebanks for Parsing Zheng-Yu Niu, Haifeng Wang, Hua Wu Toshiba (China) Research and Development Center 5/F., Tower W2, Oriental Plaza, Beijing, 100738, China {niuzhengyu,wanghaifeng,wuhua}@rdc.toshiba.com.cn Abstract We address the issue of using heteroge- neous treebanks for parsing by breaking it down into two sub-problems, convert- ing grammar formalisms of the treebanks to the same one, and parsing on these homogeneous treebanks. First we pro- pose to employ an iteratively trained tar- get grammar parser to perform grammar formalism conversion, eliminating prede- fined heuristic rules as required in previ- ous methods. Then we provide two strate- gies to refine conversion results, and adopt a corpus weighting technique for parsing on homogeneous treebanks. Results on the Penn Treebank show that our conversion method achieves 42% error reduction over the previous best result. Evaluation on the Penn Chinese Treebank indicates that a converted dependency treebank helps con- stituency parsing and the use of unlabeled data by self-training further increases pars- ing f-score to 85.2%, resulting in 6% error reduction over the previous best result. 1 Introduction The last few decades have seen the emergence of multiple treebanks annotated with different gram- mar formalisms, motivated by the diversity of lan- guages and linguistic theories, which is crucial to the success of statistical parsing (Abeille et al., 2000; Brants et al., 1999; Bohmova et al., 2003; Han et al., 2002; Kurohashi and Nagao, 1998; Marcus et al., 1993; Moreno et al., 2003; Xue et al., 2005). Availability of multiple treebanks cre- ates a scenario where we have a treebank anno- tated with one grammar formalism, and another treebank annotated with another grammar formal- ism that we are interested in. We call the first a source treebank, and the second a target tree- bank. We thus encounter a problem of how to use these heterogeneous treebanks for target gram- mar parsing. Here heterogeneous treebanks refer to two or more treebanks with different grammar formalisms, e.g., one treebank annotated with de- pendency structure (DS) and the other annotated with phrase structure (PS). It is important to acquire additional labeled data for the target grammar parsing through exploita- tion of existing source treebanks since there is of- ten a shortage of labeled data. However, to our knowledge, there is no previous study on this is- sue. Recently there have been some works on us- ing multiple treebanks for domain adaptation of parsers, where these treebanks have the same grammar formalism (McClosky et al., 2006b; Roark and Bacchiani, 2003). Other related works focus on converting one grammar formalism of a treebank to another and then conducting studies on the converted treebank (Collins et al., 1999; Forst, 2003; Wang et al., 1994; Watkinson and Manand- har, 2001). These works were done either on mul- tiple treebanks with the same grammar formalism or on only one converted treebank. We see that their scenarios …

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