Parsing : FastExact Viterbi ParseSelection
A PCFGparsingcandramaticallyreducethe time requiredto find the exact Viterbi parse by conservatively estimatingoutsideViterbi probabilities. We discussvarious estimates and give efficient algorithmsfor computing them. On Penntreebanksentences, our most detailedestimatereducesthe total numberof edgesprocessedto less than 3% of that requiredby exhausti veparsing,andevena simpler estimatewhich can be pre-computedin underaminutestill reducesthework by afactor of 5. The algorithm extendsthe classic A graphsearchprocedureto acertainhypergraphassociatedwith parsing. Unlike bestfirst and finite-beammethodsfor achieving thiskind of speed-up, theA parseris guaranteedto returnthemostlikely parse,notjustan approximation.Thealgorithmis alsocorrect for a wide rangeof parsercontrol strategies andmaintainsaworst-casecubictimebound.