1Probabilistic Context-Free Grammars (PCFGs)

• At least three ways to use probabilities in a parser – Probabilities for choosing between parses • Choose from among the many parses of the input sentence which ones are most likely – Probabilities for speedier parsing • Use probabilities to order or prune the search space of a parser for finding the best parse more quickly – Probabilities for determining the sentence • Use a parser as a language model over a word lattice in order to determine a sequence of words that has the highest probability Parsing as Search 3Parsing for Disambiguation • The integration of sophisticated structural and probabilistic models of syntax is at the very cutting edge of the field – For the non-probabilistic syntax analysis • The context-free grammar (CFG) is the standard – For the probabilistic syntax analysis • No single model has become a standard • A number of probabilistic augmentations to context-free grammars – Probabilistic CFG with the CYK algorithm – Probabilistic lexicalized CF