jurafsky&martin_3rdEd_17 (1).pdf

In the following sections well discuss in more detail

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In the following sections, we’ll discuss in more detail the inventory of relations used in dependency parsing, as well as the formal basis for these dependency struc- tures. We’ll then move on to discuss the dominant families of algorithms that are used to automatically produce these structures. Finally, we’ll discuss how to eval- uate dependency parsers and point to some of the ways they are used in language processing applications. 14.1 Dependency Relations The traditional linguistic notion of grammatical relation provides the basis for the Grammatical relation binary relations that comprise these dependency structures. The arguments to these relations consist of a head and a dependent . We’ve already discussed the notion of Head Dependent heads in Chapter 11 and Chapter 13 in the context of constituent structures. There, the head word of a constituent was the central organizing word of a larger constituent (e.g, the primary noun in a noun phrase, or verb in a verb phrase). The remaining words in the constituent are either direct, or indirect, dependents of their head. In dependency-based approaches, the head-dependent relationship is made explicit by directly linking heads to the words that are immediately dependent on them, bypass- ing the need for constituent structures. In addition to specifying the head-dependent pairs, dependency grammars allow us to further classify the kinds of grammatical relations, or grammatical function , Grammatical function
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14.1 D EPENDENCY R ELATIONS 247 Clausal Argument Relations Description NSUBJ Nominal subject DOBJ Direct object IOBJ Indirect object CCOMP Clausal complement XCOMP Open clausal complement Nominal Modifier Relations Description NMOD Nominal modifier AMOD Adjectival modifier NUMMOD Numeric modifier APPOS Appositional modifier DET Determiner CASE Prepositions, postpositions and other case markers Other Notable Relations Description CONJ Conjunct CC Coordinating conjunction Figure 14.2 Selected dependency relations from the Universal Dependency set. (de Marn- effe et al., 2014) in terms of the role that the dependent plays with respect to its head. Familiar notions such as subject , direct object and indirect object are among the kind of relations we have in mind. In English these notions strongly correlate with, but by no means de- termine, both position in a sentence and constituent type and are therefore somewhat redundant with the kind of information found in phrase-structure trees. However, in more flexible languages the information encoded directly in these grammatical rela- tions is critical since phrase-based constituent syntax provides little help. Not surprisingly, linguists have developed taxonomies of relations that go well beyond the familiar notions of subject and object. While there is considerable vari- ation from theory to theory, there is enough commonality that efforts to develop a computationally useful standard are now possible. The Universal Dependencies Universal Dependencies project (Nivre et al., 2016)
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