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We present several algorithms for assigning heads in phrase structure trees, based on different linguistic intuitions on the role of heads in natural language syntax. Starting point of our approach is the observation that a head-annotated treebank defines a unique lexicalized tree substitution grammar. This allows us to go back and forth between the two representations, and define objective functions for the unsupervised learning of head assignments in terms of features of the implicit lexicalized tree grammars. We evaluate algorithms based on the match with gold standard head-annotations, and the comparative parsing accuracy of the lexicalized grammars they give. | Unsupervised Methods for Head Assignments Federico Sangati Willem Zuidema Institute for Logic Language and Computation University of Amsterdam the Netherlands f.sangati zuidema @uva.nl Abstract We present several algorithms for assigning heads in phrase structure trees based on different linguistic intuitions on the role of heads in natural language syntax. Starting point of our approach is the observation that a head-annotated treebank defines a unique lexicalized tree substitution grammar. This allows us to go back and forth between the two representations and define objective functions for the unsupervised learning of head assignments in terms of features of the implicit lexical-ized tree grammars. We evaluate algorithms based on the match with gold standard head-annotations and the comparative 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 interannotation-standard agreement for German. On the second task the implied lex-icalized grammars score 4 points higher on parsing accuracy than lexicalized grammars derived by commonly used heuristics. 1 Introduction The head of a phrasal constituent is a central concept in most current grammatical theories and many syntax-based NLP techniques. The term is used to mark for any nonterminal node in a syntactic tree the specific daughter node that fulfills a special role however theories and applications differ widely in what that special role is supposed to be. In descriptive grammatical theories the role of the head can range from the determinant of agreement or the locus of inflections to the governor that selects the morphological form of its sister nodes or the constituent that is distributionally equivalent to its parent Corbett et al. 2006 . In computational linguistics heads mainly serve to select the lexical content on which the probability of a production should depend Char-niak 1997 Collins 1999 . With the .
