Computational complexity of probabilistic disambiguation by means of tree-grammars

Abstract

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 word-graph. The NP-hardness of computing the MPS from a word-graph also holds for Stochastic Context-Free Grammars (SCFGs).