Paper: Unsupervised Methods for Head Assignments

ACL ID E09-1080
Title Unsupervised Methods for Head Assignments
Venue Annual Meeting of The European Chapter of The Association of Computational Linguistics
Session Main Conference
Year 2009

We present several algorithms for assign- ing heads in phrase structure trees, based on different linguistic intuitions on the role of heads in natural language syntax. Start- ing point of our approach is the obser- vation that a head-annotated treebank de- fines a unique lexicalized tree substitution grammar. This allows us to go back and forth between the two representations, and define objective functions for the unsu- pervised learning of head assignments in terms of features of the implicit lexical- ized tree grammars. We evaluate algo- rithms based on the match with gold stan- dard head-annotations, and the compar- ative parsing accuracy of the lexicalized grammars they give rise to. On the first task, we approach the accuracy of hand- designed heuristics for English and inter- annot...