Automatic Transformation of the Thai Categorial Grammar Treebank to Dependency Trees

Proceedings of the 5th International Joint Conference on Natural Language Processing, pages 1243–1250, Chiang Mai, Thailand, November 8 – 13, 2011. c⃝2011 AFNLP Automatic Transformation of the Thai Categorial Grammar Treebank to Dependency Trees Christian Rishøj CST University of Copenhagen crjensen@hum.ku.dk Taneth Ruangrajitpakorn HLT Lab NECTEC taneth.rua@nectec.or.th Prachya Boonkwan HLT Lab NECTEC prachya.boo@nectec.or.th Thepchai Supnithi HLT Lab NECTEC thepchai.sup@nectec.or.th Abstract A method for deriving an approximately labeled dependency treebank from the Thai Categorial Grammar Treebank has been implemented. The method involves a lexical dictionary for assigning depen- dency directions to the CG types associ- ated with the grammatical entities in the CG bank, falling back on a generic map- ping of CG types in case of unknown words. Currently, all but a handful of the trees in the Thai CG bank can unambiguously be transformed into directed dependency trees. Dependency labels can optionally be assigned with a learned classifier, which in a preliminary evaluation with a very small training set achieves 76.5% label accuracy. In the process, a number of annotation er- rors in the CG bank were identified and corrected. Although rather limited in its coverage, excluding e.g. long-distance de- pendencies, topicalisations and longer sen- tences, the resulting treebank is believed to be sound in terms of structural annota- tional consistency and a valuable comple- ment to the scarce Thai language resources in existence. 1 Introduction Syntactic resources play an essential role for the majority of NLP applications, but for the Thai lan- guage, openly available syntactic resources are few in number: So far, the only reported resources are the CG treebank [Ruangrajitpakorn et al., 2009] and the NAiST dependency bank [Wacharaman- otham et al., 2007, Sudprasert, 2008]. Others are either unpublished or minuscule in size. Rather than relying exclusively on labor-intensive manual annotation for further expanding the resources, it would be economically sound to leverage existing efforts and transform an existing treebank in one formalism into another. 1.1 Categorial grammar Categorial grammar (CG) is a lexicalised theory in natural language syntax motivated by the principle of constitutionality and organised according to the syntactic elements [Steedman, 2000, Ajdukiewicz, 1935], and forms the theoretical basis for the Thai CG treebank. The resource building effort has been very fruitful, but there remains phenomena of Thai language, including long-distance dependen- cies and topicalisation [Warotamasikkhadit, 1997], which are unhandled by the instantiation of CG currently in use. Additionally, although the Thai language belongs to a fixed word order typology, Thai spoken language exhibits some flexibility in word order, due to the occasional preference of Thai language users for correspondence in rhyme. As an example, consider the following sentence1: อังกฤษ คิดคน อยาง หนัก วัคซีน ปองกันเชื้อ ไวรัส ไขหวัดนก (Lit: England/NE invent/V “-ly”/ADVPFX heavy/ADJ vaccine/N protect/V virus/N avian_flu/NP) “The British are strenuously developing an Avian Flu vaccine.” The adverbial compound formed by อยาง หนัก (“strenuously”) conventionally occurs after the di- rect object, but is in this sentence, it is realised in the pre-direct object position2 in order for the last syllable [ nakL] of อยาง หนัก …

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