jurafsky&martin_3rdEd_17 (1).pdf

The san francisco examiner in fig 225 the governing

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The San Francisco Examiner in Fig. 22.5 ): The governing predicate , in this case the verb issued . The predicate is a cru- cial feature since labels are defined only with respect to a particular predicate. The phrase type of the constituent, in this case, NP (or NP-SBJ ). Some se- mantic roles tend to appear as NP s, others as S or PP , and so on.
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386 C HAPTER 22 S EMANTIC R OLE L ABELING S NP-SBJ = ARG0 VP DT NNP NNP NNP The San Francisco Examiner VBD = TARGET NP = ARG1 PP-TMP = ARGM-TMP issued DT JJ NN IN NP a special edition around NN NP-TMP noon yesterday Figure 22.5 Parse tree for a PropBank sentence, showing the PropBank argument labels. The dotted line shows the path feature NP " S # VP # VBD for ARG0, the NP-SBJ constituent The San Francisco Examiner. The headword of the constituent, Examiner . The headword of a constituent can be computed with standard head rules, such as those given in Chapter 11 in Fig. 11.12 . Certain headwords (e.g., pronouns) place strong constraints on the possible semantic roles they are likely to fill. The headword part of speech of the constituent, NNP . The path in the parse tree from the constituent to the predicate. This path is marked by the dotted line in Fig. 22.5 . Following Gildea and Jurafsky (2000) , we can use a simple linear representation of the path, NP " S # VP # VBD. " and # represent upward and downward movement in the tree, respectively. The path is very useful as a compact representation of many kinds of grammatical function relationships between the constituent and the predicate. The voice of the clause in which the constituent appears, in this case, active (as contrasted with passive ). Passive sentences tend to have strongly different linkings of semantic roles to surface form than do active ones. The binary linear position of the constituent with respect to the predicate, either before or after . The subcategorization of the predicate, the set of expected arguments that appear in the verb phrase. We can extract this information by using the phrase- structure rule that expands the immediate parent of the predicate; VP ! VBD NP PP for the predicate in Fig. 22.5 . The named entity type of the constituent. The first words and the last word of the constituent. The following feature vector thus represents the first NP in our example (recall that most observations will have the value NONE rather than, for example, ARG 0, since most constituents in the parse tree will not bear a semantic role): ARG 0: [issued, NP, Examiner, NNP, NP " S # VP # VBD, active, before, VP ! NP PP, ORG, The, Examiner] Other features are often used in addition, such as sets of n-grams inside the constituent, or more complex versions of the path features (the upward or downward halves, or whether particular nodes occur in the path). It’s also possible to use dependency parses instead of constituency parses as the basis of features, for example using dependency parse paths instead of constituency paths.
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22.7 S ELECTIONAL R ESTRICTIONS 387 Further Issues in Semantic Role Labeling
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