TAILIEUCHUNG - Báo cáo khoa học: "Constraints on Non-Projective Dependency Parsing"

We investigate a series of graph-theoretic constraints on non-projective dependency parsing and their effect on expressivity, . whether they allow naturally occurring syntactic constructions to be adequately represented, and efficiency, . 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. . | Constraints on Non-Projective Dependency Parsing Joakim Nivre Vaxjo University School of Mathematics and Systems Engineering Uppsala University Department of Linguistics and Philology Abstract We investigate a series of graph-theoretic constraints on non-projective dependency parsing and their effect on expressivity . whether they allow naturally occurring syntactic constructions to be adequately represented and efficiency . 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 of the dependency graphs found in the two treebanks. This is a substantial improvement over the commonly used projective approximation degree 0 which excludes 15-25 of the graphs. 1 Introduction Data-driven approaches to syntactic parsing has until quite recently been limited to representations that do not capture non-local dependencies. This is true regardless of whether representations are based on constituency where such dependencies are traditionally represented by empty categories and coindexation to avoid explicitly discontinuous constituents or on dependency where it is more common to use a direct encoding of so-called non-projective dependencies. While this surface dependency approximation Levy and Manning 2004 may be acceptable for certain applications of syntactic parsing it is clearly not adequate as a basis for deep semantic interpretation which explains the growing body of research devoted to different methods for .

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