Unlexicalized Transition-basedDiscontinuous Constituency Parsing
Table 4: Discontinuous parsing results on the development sets.
['Transition System', 'Features', 'Oracle', 'English F', 'English Disc. F', 'German (Tiger) F', 'German (Tiger) Disc. F', 'German (Negra) F', 'German (Negra) Disc. F']
5.4.1 Effect of Lexicalization The former consistently obtains higher results. The F-score difference is small on English (0.1 to 0.3) but substantial on the German treebanks (more than 1.0 absolute point) and in general on discontinuous constituents (Disc. F). First of all, the parser tends to be conservative when predicting discontinuities, there are in general few false positives. Instead, the scarcity of discontinuities in the data might be a determining factor: only 20% of sentences in the Discontinuous Penn Treebank contain at least one discontinuity and 30% of sentences in the Negra and Tiger corpus.