# PROBABILISTIC CFGs & PROBABILISTIC PARSING - PowerPoint PPT Presentation

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PROBABILISTIC CFGs & PROBABILISTIC PARSING. Universita’ di Venezia 3 Ottobre 2003. Probabilistic CFGs. Context-Free Grammar Rules are of the form: S  NP VP In a Probabilistic CFG, we assign a probability to these rules: S  NP VP, P(SNP,VP|S). Why PCFGs?.

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PROBABILISTIC CFGs & PROBABILISTIC PARSING

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## PROBABILISTIC CFGs &PROBABILISTIC PARSING

Universita’ di Venezia

3 Ottobre 2003

### Probabilistic CFGs

• Context-Free Grammar Rules are of the form:

• S  NP VP

• In a Probabilistic CFG, we assign a probability to these rules:

• S  NP VP, P(SNP,VP|S)

### Why PCFGs?

DISAMBIGUATION: with a PCFG, probabilities can be used to choose the most likely parse

ROBUSTNESS: rather than excluding things, a PCFG may assign them a very low probability

LEARNING: CFGs cannot be learned from positive data only

### PCFGs in Prolog (courtesy Doug Arnold)

s(P0, [s,NP,VP] ) --> np(P1,NP),vp(P2,VP),{ P0 is 1.0*P1*P2 }.

….vp(P0, [vp,V,NP] ) -->v(P1,V),np(P2,NP ),{ P0 is 0.7*P1*P2 }.

### Independence assumptions

PCFGs specify a language model, just like n-grams

We need however to make some independence assumptions yet again: the probability of a subtree is independent of:

### Parsing with PCFGs:A comparison with HMMs

An HMM defines a REGULAR GRAMMAR:

### Learning probabilities

Reconstruct the rules used in the analysis of the Treebank

Estimate probabilities by:P(AB) = C(AB) / C(A)