Cross-Lingual Syntactic Transfer with Limited Resources

Cross-Lingual Syntactic Transfer with Limited Resources Mohammad Sadegh Rasooli and Michael Collins∗ Department of Computer Science, Columbia University New York, NY 10027, USA {rasooli,mcollins}@cs.columbia.edu Abstract We describe a simple but effective method for cross-lingual syntactic transfer of depen- dency parsers, in the scenario where a large amount of translation data is not available. This method makes use of three steps: 1) a method for deriving cross-lingual word clus- ters, which can then be used in a multilingual parser; 2) a method for transferring lexical information from a target language to source language treebanks; 3) a method for integrat- ing these steps with the density-driven annota- tion projection method of Rasooli and Collins (2015). Experiments show improvements over the state-of-the-art in several languages used in previous work, in a setting where the only source of translation data is the Bible, a con- siderably smaller corpus than the Europarl corpus used in previous work. Results using the Europarl corpus as a source of translation data show additional improvements over the results of Rasooli and Collins (2015). We con- clude with results on 38 datasets from the Uni- versal Dependencies corpora. 1 Introduction Creating manually-annotated syntactic treebanks is an expensive and time consuming task. Recently there has been a great deal of interest in cross-lingual syntactic transfer, where a parsing model is trained for some language of interest, using only treebanks in other languages. There is a clear motivation for this in building parsing models for languages for which treebank data is unavailable. Methods ∗On leave at Google Inc. New York. for syntactic transfer include annotation projection methods (Hwa et al., 2005; Ganchev et al., 2009; McDonald et al., 2011; Ma and Xia, 2014; Rasooli and Collins, 2015; Lacroix et al., 2016; Agi´c et al., 2016), learning of delexicalized models on univer- sal treebanks (Zeman and Resnik, 2008; McDon- ald et al., 2011; T¨ackstr¨om et al., 2013; Rosa and Zabokrtsky, 2015), treebank translation (Tiedemann et al., 2014; Tiedemann, 2015; Tiedemann and Agi´c, 2016) and methods that leverage cross-lingual rep- resentations of word clusters, embeddings or dictio- naries (T¨ackstr¨om et al., 2012; Durrett et al., 2012; Duong et al., 2015a; Zhang and Barzilay, 2015; Xiao and Guo, 2015; Guo et al., 2015; Guo et al., 2016; Ammar et al., 2016a). This paper considers the problem of cross-lingual syntactic transfer with limited resources of mono- lingual and translation data. Specifically, we use the Bible corpus of Christodouloupoulos and Steed- man (2014) as a source of translation data, and Wikipedia as a source of monolingual data. We de- liberately limit ourselves to the use of Bible trans- lation data because it is available for a very broad set of languages: the data from Christodouloupou- los and Steedman (2014) includes data from 100 languages. The Bible data contains a much smaller set of sentences (around 24,000) than other transla- tion corpora, for example Europarl (Koehn, 2005), which has around 2 million sentences per language pair. This makes it a considerably more challeng- ing corpus to work with. …

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