G22.2590 - Natural Language Processing - Spring 2001
Lecture 11 Outline
Prof. Grishman
April 4, 2001
Resolving Anaphoric Reference, cont’d (Allen Ch. 14)
Pronominal coreference
General strategy for resolving pronouns:
- generate discourse entities for prior sentences in a discourse
- apply constraints to select best candidate antecedent for pronoun
Computing discourse entities for noun phrases
- proper names and definite NPs (“Fred met the president of NYU.”)
- indefinite NPs (“Fred ate an apple.”)
- indefinite NPs in scope of universal quantifiers (“Every boy ate an apple.”)
Constraints:
recency (prefer most recent antecedent; exclude antecedents beyond a certain
distance)
focus / centering
prefer current focus of discourse
prefer argument of main clause of prior sentence; prefer subject to object
breadth-first search provides good approximation (Hobbs)
number (for it vs. they)
note complication for indefinite NPs inside universal quantifiers
animate (for he/she vs. it)
gender (for he vs. she)
requires determination of gender for antecedents
reflexive / non-reflexive pronouns
selectional constraints
does antecedent satisfy selectional constraints of context of anaphor?
Other forms of anaphora
Definite descriptions (Allen 14.4)
May refer to entities from earlier discourse contexts (not just local context), or even
to uniquely determined entities in the setting of the discourse.
In addition, we may make use of background knowledge (for example, about partwhole relations) to make definite references which introduce objects related to
objects in the discourse history. For example, “I bought a new frying pan. The
lid of the pan was made of glass.” Or “The winner of yesterday’s 5-mile race got a
new car.” The definite reference requires (or implies) that the object is unique.
In many cases, the reference to the prior object is implicit (Allen, p. 443): ““I bought
a new frying pan. The lid was made of glass.” Or “I saw an exciting 5-mile race
yesterday. The winner got a new car.” In such cases, if we can identify the
semantic class of the implicit element (null anaphor), we can use conventional
(pronominal) anaphora resolution methods to retrieve the referent.
‘One’ (surface) anaphora (Allen 14.7)
“I bought a red car. Mary bought a green one.” is an example of a quite different
anaphor phenomenon. Here we refer not to a specific entity evoked by the prior
discourse, but to (parts of) a description of such an entity. It is therefore sometimes
called “description anaphora.” While the process of identifying the antecedent is
generally similar, the way in which the antecedent is used is quite different: one
takes just those parts of the antecedent which do not conflict with the modifiers in the
anaphor (we do not use the discourse entities). This may be done at the surface level
(rather than at the logical form level).
Ellipsis (Allen 14.6)
Ellipsis involves input which is not a full clause / sentence, but can be ‘filled out’
with reference to prior sentences. Sometimes this is indicated by a bare auxiliary:
“John passed the course. Mary did too.” Sometimes it takes the form of a bare noun
phrase, particularly in dialog: “Would you like some chocolate crunch? No. Apple
walnut?”
Resolving ellipsis involves identifying the antecedent constituent, and then
determining which parts are to be replaced by the current phrase. Sometimes this can
be done syntactically; sometimes semantic constraints are essential, particularly for
analyzing bare NPs. Some ellipsis mechanism is essential for analyzing dialog.
Anaphora resolution mechanism in Jet (separate handout)
In the example patterns, we use the anaphora mechanism to recover the company
name for appoint events.
In contrast to the earlier pattern sets, we generate an explicit event annotation for each
appointment event; we fill the company slot with an empty noun group with head
‘company’, which will be resolved to the actual company name (usually) by reference
resolution.
Assignment #10 (the last): due April 11th:
.
Exercises #2, 7, and 8 from Chapter 14.