Elimination of Spurious Ambiguity in Transition-Based Dependency Parsing

In parsing, spurious ambiguity refers to ambiguity in a grammar that occurs because several derivations exist for an identical syntactic analysis. When the grammar is enriched with probabilities, the existence of spurious ambiguity implies that the statistical model is defined over derivations, a more fine-grained version of the actual syntactic structures of interest. The probability of a syntactic structure then becomes the marginalized probability over all derivations that map to that syntactic structure. Spurious ambiguity can exist in various grammatical models such as combinatory categorial grammars [Steedman, 2001], tree adjoining grammars [Joshi et al., 1975], data-oriented parsing [Bod, 1992] and transition-based dependency parsing [Nivre, 2005].