Exploring Syntactic Structural Features for Sub-Tree Alignment Using Bilingual Tree Kernels

Proceedings of the 48th Annual Meeting of the Association for Computational Linguistics, pages 306–315, Uppsala, Sweden, 11-16 July 2010. c⃝2010 Association for Computational Linguistics Exploring Syntactic Structural Features for Sub-Tree Alignment using Bilingual Tree Kernels Jun Sun1,2 Min Zhang1 Chew Lim Tan2 1 Institute for Infocomm Research 2School of Computing, National University of Singapore sunjun@comp.nus.edu.sg mzhang@i2r.a-star.edu.sg tancl@comp.nus.edu.sg Abstract We propose Bilingual Tree Kernels (BTKs) to capture the structural similarities across a pair of syntactic translational equivalences and apply BTKs to sub-tree alignment along with some plain features. Our study reveals that the struc- tural features embedded in a bilingual parse tree pair are very effective for sub-tree alignment and the bilingual tree kernels can well capture such features. The experimental results show that our approach achieves a significant im- provement on both gold standard tree bank and automatically parsed tree pairs against a heuris- tic similarity based method. We further apply the sub-tree alignment in machine translation with two methods. It is suggested that the sub- tree alignment benefits both phrase and syntax based systems by relaxing the constraint of the word alignment. 1 Introduction Syntax based Statistical Machine Translation (SMT) systems allow the translation process to be more grammatically performed, which provides decent reordering capability. However, most of the syntax based systems construct the syntactic trans- lation rules based on word alignment, which not only suffers from the pipeline errors, but also fails to effectively utilize the syntactic structural fea- tures. To address those deficiencies, Tinsley et al. (2007) attempt to directly capture the syntactic translational equivalences by automatically con- ducting sub-tree alignment, which can be defined as follows: A sub-tree alignment process pairs up sub-tree pairs across bilingual parse trees whose contexts are semantically translational equivalent. Accord- ing to Tinsley et al. (2007), a sub-tree aligned parse tree pair follows the following criteria: (i) a node can only be linked once; (ii) descendants of a source linked node may only link to descendants of its target linked counterpart; (iii) ancestors of a source linked node may on- ly link to ancestors of its target linked counterpart. By sub-tree alignment, translational equivalent sub-tree pairs are coupled as aligned counterparts. Each pair consists of both the lexical constituents and their maximum tree structures generated over the lexical sequences in the original parse trees. Due to the 1-to-1 mapping between sub-trees and tree nodes, sub-tree alignment can also be consi- dered as node alignment by conducting multiple links across the internal nodes as shown in Fig. 1. Previous studies conduct sub-tree alignments by either using a rule based method or conducting some similarity measurement only based on lexi- cal features. Groves et al. (2004) conduct sub-tree alignment by using some heuristic rules, lack of extensibility and generality. Tinsley et al. (2007) Figure 1: Sub-tree alignment as referred to Node alignment 306 and Imamura (2001) propose some score functions based on the lexical similarity and co-occurrence. These works fail to utilize the structural features, rendering the syntactic rich task of sub-tree align- …

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