Robust Constituent-to-Dependency Conversion for English

This paper suggests a robust way of converting constituent-based trees in the Penn Treebank style into dependency trees for several different English corpora. For English, there already exist conversion tools. However, these tools are often customized enough for a specific corpus that they do not necessarily work as well when applied to different corpora involving newly introduced POS-tags or annotation schemes. The desire to improve conversion portability motivated us to build a new conversion tool that would produce more robust results across different corpora. In particular, we have modified the treatment of head-percolation rules, function tags, coordination, gapping, and empty category mappings. We compare our method with the LTH conversion tool used for the CoNLL’07-09 shared tasks. For our experiments, we use 6 different English corpora from OntoNotes release 4.0. To demonstrate the impact our approach has on parsing, we train and test two state-of-the-art dependency parsers, MaltParser and MSTParser, and our own parser, ClearParser, using converted output from both the LTH tool and our method. Our results show that our method removes certain unnecessary nonprojective dependencies and generates fewer unclassified dependencies. All three parsers give higher parsing accuracies on average across these corpora using data generated by our method; especially on semantic dependencies.