Constraints on Non-Projective Dependency Parsing

We investigate a series of graph-theoretic constraints on non-projective dependency parsing and their effect on expressivity, i.e. whether they allow naturally occurring syntactic constructions to be adequately represented, and efficiency, i.e. whether they reduce the search space for the parser. In particular, we define a new measure for the degree of non-projectivity in an acyclic dependency graph obeying the single-head constraint. The constraints are evaluated experimentally using data from the Prague Dependency Treebank and the Danish Dependency Treebank. The results indicate that, whereas complete linguistic coverage in principle requires unrestricted non-projective dependency graphs, limiting the degree of non-projectivity to at most 2 can reduce average running time from quadratic to linear, while excluding less than 0.5% of the dependency graphs found in the two treebanks. This is a substantial improvement over the commonly used projective approximation (degree 0), which excl...

• Type: Conference
• Year: 2006
• Where: EACL
• Authors: Joakim Nivre
• Keywords: Dependency Graphs, Dependency Treebank, EACL 2006, Natural Language Processing, Non-projective Dependency