Computational Complexity of Probabilistic Disambiguation by means of Tree-Grammars

This paper studies the computational complexity of disambiguation under probabilistic tree-grammars as in (Bod, 1992; Schabes and Waters, 1993). It presents a proof that the following problems are NP-hard: computing the Most Probable Parse from a sentence or from a word-graph, and computing the Most Probable Sentence (MPS) from a wordgraph. The NP-hardness of computing the MPS from a word-graph also holds for Stochastic Context-Free Grammars (SCFGs). 1 Motivation Statistical disambiguation is currently a popular technique in parsing Natural Language. Among the models that implement statistical disambiguation one nds the models that employ Tree-Grammars such as Data Oriented Parsing (DOP) (Scha, 1990; Bod, 1992) and Stochastic (Lexicalized) Tree-Adjoining Grammar (STAG) (Schabes and Waters, 1993). These models extend the domain of locality for expressing constraints from simple Context-Free Grammar (CFG) productions to deeper structures called elementary-trees. Due to this extension, t...

• Type: Conference
• Year: 1996
• Where: COLING
• Authors: Khalil Sima'an
• Keywords: COLING 1996, COLING 2008, Context-free Grammar, Most Probable Parse, Statistical Disambiguation