Mildly non-projective dependency structures

Abstract

Syntactic parsing requires a fine balance between expressivity and complexity, so that naturally occurring structures can be accurately parsed without compromising efficiency. In dependency-based parsing, several constraints have been proposed that restrict the class of permissible structures, such as projectivity, planarity, multi-planarity, well-nestedness, gap degree, and edge degree. While projectivity is generally taken to be too restrictive for natural language syntax, it is not clear which of the other proposals strikes the best balance between expressivity and complexity. In this paper, we review and compare the different constraints theoretically, and provide an experimental evaluation using data from two treebanks, investigating how large a proportion of the structures found in the treebanks are permitted under different constraints. The results indicate that a combination of the well-nestedness constraint and a parametric constraint on discontinuity gives a very good fit with the linguistic data.

Scarcity of crossing dependencies: A direct outcome of a specific constraint?

“…However, while some classes of dependency structures tolerating certain crossings have a very good empirical coverage [31,[42][43][44], these proposals still face counterexamples that fall outside the restrictions [45][46][47].…”

Dependency Structures Derived from Minimalist Grammars

“…[9] define an interval as the set [i, j] := {k ∈ V |i ≤ k and k ≤ j}, where i and j are endpoints, and V is a set of nodes as defined in Section 3.1. Non-projective structures violate this constraint.…”

Bilingual Structured Language Models for Statistical Machine Translation

“…Finally, we make a projectivity assumption, which is supported by empirical data in many languages (Kuhlmann and Nivre, 2006;Havelka, 2007), and makes a model computationally less expensive. A dependency parse D of a sentence W = w 1 , .…”