lect18-unsup.ppt - Statistical Models of Semantics and...

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Unformatted text preview: Statistical Models of Semantics and Unsupervised Language Discovery Lecture #18 Introduction to Natural Language Processing CMPSCI 585, Fall 2007 Andrew McCallum Computer Science Department University of Massachusetts Amherst Including slides from Chris Manning, Dan Klein, Rion Snow & Patrick Pantel. Attachment Ambiguity • Where to attach a phrase in the parse tree? • “I saw the man with the telescope.” – What does “with a telescope” modify? – Is the problem AI complete? Yes, but… – Proposed simple structural factors • Right association [Kimball 1973] ‘low’ or ‘near’ attachment = ‘early closure’ of NP • Minimal attachment [Frazier 1978] (depends on grammar) = ‘high’ or ‘distant’ attachment = ‘late closure’ (of NP) Attachment Ambiguity • “The children ate the cake with a spoon . ” • “The children ate the cake with frosting .” • “Joe included the package for Susan .” • “Joe carried the package for Susan .” • Ford, Bresnan and Kaplan (1982): “It is quite evident, then, that the closure effects in these sentences are induced in some way by the choice of the lexical items.” Lexical acquisition, semantic similarity • Previous models give same estimate to all unseen events. • Unrealistic - could hope to refine that based on semantic classes of words • Examples – “Susan ate the cake with a durian.” – “Susan had never eaten a fresh durian before.” – Although never seen “eating pineapple” should be more likely than “eating holograms” because pineapple is similar to apples, and we have seen “eating apples”. An application: selectional preferences • Most verbs prefer arguments of a particular type. Such regularities are called selectional preferences or selectional restrictions . • “Bill drove a…” Mustang, car, truck, jeep • Selectional preference strength: how strongly does a verb constrain direct objects • “see” versus “unknotted” Measuring selectional preference strength • Assume we are given a clustering of (direct object) nouns. Resnick (1993) uses WordNet. • Selectional association between a verb and a class Proportion that its summand contributes to preference strength. • For nouns in multiple classes, disambiguate as most likely sense: Selection preference strength (made up data) Noun class c P(c) P(c|eat) P(c|see) P(c|find) people 0.25 0.01 0.25 0.33 furniture 0.25 0.01 0.25 0.33 food 0.25 0.97 0.25 0.33 action 0.25 0.01 0.25 0.01 SPS S(v) 1.76 0.00 0.35 A(eat, food) = 1.08 A(find, action) = -0.13 Selectional Preference Strength example (Resnick, Brown corpus) But how might we measure word similarity for word classes?...
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This note was uploaded on 02/22/2012 for the course CMPSCI 585 taught by Professor Staff during the Fall '08 term at UMass (Amherst).

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lect18-unsup.ppt - Statistical Models of Semantics and...

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