Hi, George: The probability in PCFG (Probabilistic Context Free Grammer) is used for disambiguate competing parsings. So its literal value (here the 0.00072) is not that important. Normally, if a sentence has more words, the total parsing probability of it tends to be smaller. So it is normally not possible to compare the probability across sentences.
In most real sentences, the possible number of different parses is normally from several tens to thousands. So the probability plays a good role (the accuracy of PCFG parser is about 70% in WSJ treebank). Best regards Yimin Zhang Intel China Research Center - -----Original Message----- From: George Paliouras [mailto:paliourg@;iit.demokritos.gr] Sent: 2002年10月31日 1:05 To: [EMAIL PROTECTED] Subject: [UAI] Probabilities in CFGs Dear all, I have a question about the use of probabilities in (context-free) grammars. According to common use, the probability of a specific parse of sentence is calculated as the product of probabilities of all the rules involved in the parse tree. Usually, the probabilities that are assigned to the rules are calculated from the frequency by which each rule participates in the "correct" parse trees of the training sentences. Thus, using the following simple grammar (rule probabilities in brackets): S -> NP VP (1.0) NP -> ART NOUN (0.3) NP -> ART NOUN RC (0.7) VP -> VERB NP (1.0) RC -> that VP (1.0) VERB -> saw (0.4) VERB -> heard (0.6) NOUN -> cat (0.2) NOUN -> dog (0.4) NOUN -> mouse (0.4) ART -> a (0.5) ART -> the (0.5) to parse the sentence "the cat saw the mouse", one gets the probability: 1.0*0.3*0.5*0.2*1.0*0.4*0.3*0.5*0.4 = 0.00072, as the sentence has the following parse tree: (S (NP (ART the) (NOUN cat)) (VP (VERB saw)) (NP (ART the) (NOUN mouse))) This approach seems "generative" in the sense that the calculated probability corresponds to the probability of the sentence being generated by the grammar. However, the significance of this number in a "parsing" mode is not clear to me. A bottom-up parser would be able to generate the above tree *unambiguously*, i.e., there is no other way to parse this sentence with the given grammar. Therefore it seems reasonable to arrive at the probability estimate of 1.0 for the parse. This could be achieved by the use of a different approach to the calculation of rule probabilities. Namely, assign a probability <1.0 only when two rules share the same body (and of course have different heads), that is only when there is ambiguity on what rule to use for parsing. Given that I have not met this approach in the literature, I assume that something is wrong in my reasoning. Any help on this issue and references to related work would be greately appreciated. Tanks in advance, George Paliouras
