Topological Field Parsing of German

Proceedings of the 47th Annual Meeting of the ACL and the 4th IJCNLP of the AFNLP, pages 64–72, Suntec, Singapore, 2-7 August 2009. c⃝2009 ACL and AFNLP Topological Field Parsing of German Jackie Chi Kit Cheung Department of Computer Science University of Toronto Toronto, ON, M5S 3G4, Canada jcheung@cs.toronto.edu Gerald Penn Department of Computer Science University of Toronto Toronto, ON, M5S 3G4, Canada gpenn@cs.toronto.edu Abstract Freer-word-order languages such as Ger- man exhibit linguistic phenomena that present unique challenges to traditional CFG parsing. Such phenomena produce discontinuous constituents, which are not naturally modelled by projective phrase structure trees. In this paper, we exam- ine topological field parsing, a shallow form of parsing which identifies the ma- jor sections of a sentence in relation to the clausal main verb and the subordinat- ing heads. We report the results of topo- logical field parsing of German using the unlexicalized, latent variable-based Berke- ley parser (Petrov et al., 2006) Without any language- or model-dependent adapta- tion, we achieve state-of-the-art results on the T¨uBa-D/Z corpus, and a modified NE- GRA corpus that has been automatically annotated with topological fields (Becker and Frank, 2002). We also perform a qual- itative error analysis of the parser output, and discuss strategies to further improve the parsing results. 1 Introduction Freer-word-order languages such as German ex- hibit linguistic phenomena that present unique challenges to traditional CFG parsing. Topic focus ordering and word order constraints that are sen- sitive to phenomena other than grammatical func- tion produce discontinuous constituents, which are not naturally modelled by projective (i.e., with- out crossing branches) phrase structure trees. In this paper, we examine topological field parsing, a shallow form of parsing which identifies the ma- jor sections of a sentence in relation to the clausal main verb and subordinating heads, when present. We report the results of parsing German using the unlexicalized, latent variable-based Berkeley parser (Petrov et al., 2006). Without any language- or model-dependent adaptation, we achieve state- of-the-art results on the T¨uBa-D/Z corpus (Telljo- hann et al., 2004), with a F1-measure of 95.15% using gold POS tags. A further reranking of the parser output based on a constraint involv- ing paired punctuation produces a slight additional performance gain. To facilitate comparison with previous work, we also conducted experiments on a modified NEGRA corpus that has been automat- ically annotated with topological fields (Becker and Frank, 2002), and found that the Berkeley parser outperforms the method described in that work. Finally, we perform a qualitative error anal- ysis of the parser output on the T¨uBa-D/Z corpus, and discuss strategies to further improve the pars- ing results. German syntax and parsing have been studied using a variety of grammar formalisms. Hocken- maier (2006) has translated the German TIGER corpus (Brants et al., 2002) into a CCG-based treebank to model word order variations in Ger- man. Foth et al. (2004) consider a version of de- pendency grammars known as weighted constraint dependency grammars for parsing German sen- tences. On the NEGRA corpus (Skut et al., …

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