Coreference

In linguistics, coreference, sometimes written co-reference, occurs when two or more expressions in a text refer to the same person or thing; they have the same referent, e.g. Bill said he would come; the proper noun Bill and the pronoun he refer to the same person, namely to Bill.[1] Coreference is the main concept underlying binding phenomena in the field of syntax. The theory of binding explores the syntactic relationship that exists between coreferential expressions in sentences and texts. When two expressions are coreferential, the one is usually a full form (the antecedent) and the other is an abbreviated form (a proform or anaphor). Linguists use indices to show coreference, as with the i index in the example Billi said hei would come. The two expressions with the same reference are coindexed, hence in this example Bill and he are coindexed, indicating that they should be interpreted as coreferential.

Types

When exploring coreference, there are numerous distinctions that can be made, e.g. anaphora, cataphora, split antecedents, coreferring noun phrases, etc.[2] When dealing with proforms (pronouns, pro-verbs, pro-adjectives, etc.), one distinguishes between anaphora and cataphora. When the proform follows the expression to which it refers, anaphora is present (the proform is an anaphor), and when it precedes the expression to which it refers, cataphora is present (the proform is a cataphor). These notions are illustrated as follows:

Anaphora
a. The musici was so loud that iti couldn't be enjoyed. –The anaphor it follows the expression to which it refers (its antecedent).
b. Our neighborsi dislike the music. If theyi are angry, the cops will show up soon. – The anaphor they follows the expression to which it refers (its antecedent).
Cataphora
a. If theyi are angry about the music, the neighborsi will call the cops. – The cataphor they precedes the expression to which it refers (its postcedent).
b. Despite heri difficulty, Wilmai came to understand the point. – The cataphor her precedes the expression to which it refers (its postcedent)
Split antecedents
a. Caroli told Bobi to attend the party. Theyi arrived together. – The anaphor they has a split antecedent, referring to both Carol and Bob.
b. When Caroli helps Bobi and Bobi helps Caroli, theyi can accomplish any task. – The anaphor they has a split antecedent, referring to both Carol and Bob.
Coreferring noun phrases
a. The project leaderi is refusing to help. The jerki thinks only of himself. – Coreferring noun phrases, whereby the second noun phrase is a predication over the first.
b. Some of our colleagues1 are going to be supportive. These kinds of people1 will earn our gratitude. – Coreferring noun phrases, whereby the second noun phrase is a predication over the first.

Versus bound variables

Semanticists and logicians sometimes draw a distinction between coreference and what is known as a bound variable.[3] An instance of a bound variable can look like coreference, but from a technical standpoint, one can argue that it actually is not. Bound variables occur when the antecedent to the proform is an indefinite quantified expression, e.g.[4]

a. Every studenti has received hisi grade. – The pronoun his is an example of a bound variable
b. No studenti was upset with hisi grade. – The pronoun his is an example of a bound variable

Quantified expressions such as every student and no student are, from a technical standpoint, not referential. The subjects every student and no student are grammatically singular, but they do not pick out single referents in the discourse world. Thus since the antecedents to the possessive adjective his is not referential, one also cannot say that his is referential. Instead, one says it is a variable that is bound by its antecedent. Its reference varies based upon which of the students in the discourse world is thought of. If Jack, John, and Jerry are the three students in the discourse world, then the meaning of his varies based upon whether Jack, John, or Jerry is the focus of the minds eye. The existence of bound variables is perhaps more apparent with the following example:

c. Only Jacki likes hisi grade. – The pronoun his can be a bound variable.

This sentence is ambiguous. It can mean that Jack likes his grade, but everyone else dislikes Jack's grade, or more likely, it means that Jack likes his grade, but John dislikes his (John's) grade, and Jerry dislikes his (Jerry's) grade. The second, more natural reading is the bound variable reading. While the distinction between coreference and bound variables may be real, coindexation can be construed as accommodating both. That is, when two or more expressions are coindexed, it indicates that one is dealing with coreference or a bound variable.

Coreference resolution

In computational linguistics, coreference resolution is a well-studied problem in discourse. To derive the correct interpretation of a text, or even to estimate the relative importance of various mentioned subjects, pronouns and other referring expressions must be connected to the right individuals. Algorithms intended to resolve coreferences commonly look first for the nearest preceding individual that is compatible with the referring expression. For example, she might attach to a preceding expression such as the woman or Anne, but not to Bill. Pronouns such as himself have much stricter constraints. As with many linguistic tasks, there is a tradeoff between precision and recall, the calculation of which can vary as no single algorithm exists to measure the quality of coreference chains. Cluster quality metrics commonly used to evaluate coreference resolution algorithms are Rand index, adjusted Rand index or different mutual information-based methods.

A classic problem for coreference resolution in English is the pronoun it, which has many uses. It can refer much like he and she, except that it generally refers to inanimate objects (the rules are actually more complex: animals may be any of it, he, or she; ships are traditionally she; hurricanes are usually it despite having gendered names). It can also refer to abstractions rather than beings, e.g. He was paid minimum wage, but didn't seem to mind it. Finally, it also has pleonastic uses, which do not refer to anything specific:

a. It's raining.
b. It's really a shame.
c. It takes a lot of work to succeed.
d. Sometimes it's the loudest who have the most influence.

Pleonastic uses are not considered referential, and so are not part of coreference.[5]

Approaches to coreference resolution can broadly be separated into mention-pair, mention-ranking or entity-based algorithms. Mention-pair algorithms involve binary decisions if a pair of two given mentions belong to the same entity. Entity-wide constraints like gender are not considered, which leads to error propagation. For example the pronouns he or she can both have a high probability of coreference with the teacher, but cannot be coreferent with each other. Mention-ranking algorithms expand on this idea but instead stipulate that one mention can only be coreferent with one (previous) mention. As a result, each previous mention must be given assigned a score and the highest scoring mention (or no mention) is linked. Finally, in entity-based methods mentions are linked based on information of the whole coreference chain instead of individual mentions. The representation of a variable-width chain is more complex and computationally expensive than mention-based methods, which lead to these algorithms being mostly based on neural network architectures.

See also
  • Anaphora (linguistics) – Use of an expression whose interpretation depends on context
  • Antecedent – Expression that gives its meaning to a pro-form in grammar
  • Binding – The distribution of anaphoric elements
  • Cataphora – Use of an expression or word that co-refers with a later, more specific, expression
  • Nearest referent
  • Switch-reference – Any clause-level morpheme that signals whether certain prominent arguments in 'adjacent' clauses are coreferential
  • Word-sense disambiguation – Identification of the sense of a word that is being used

Notes
  1. For definitions of coreference, see for instance Crystal (1997:94) and Radford (2004:332).
  2. These distinctions (anaphora, cataphora, split antecedents, coreferring noun phrases, etc.) are discussed in Jurafsky and Martin (2000:669ff.).
  3. For discussions of bound variables, see for instance Portner (2005:102ff.).
  4. See Jurafsky and Martin (2000:701) for an example of a bound variable like the ones given here.
  5. Li et al. (2009) have demonstrated high accuracy in sorting out pleonastic it, and this success promises to improve the accuracy of coreference resolution overall.

References
  • Crystal, D. 1997. A dictionary of linguistics and phonetics. 4th edition. Cambridge, MA: Blackwell Publishing.
  • Jurafsky, D. and H. Martin 2000. Speech and language processing: An introduction to natural language processing, computational linguistics, and speech recognition. New Delhi, India: Pearson Education.
  • Portner, P. 2005. What is semantics?: Fundamentals of formal semantics. Malden, MA: Blackwell Publishing.
  • Radford, A. 2004. English syntax: An introduction. Cambridge, UK: Cambridge University Press.
  • Yifan, L., P. Musilek, M. Reformat, and L. Wyard-Scott 2009. Identification of pleonastic it using the web. Journal of Artificial Intelligence Research 34, 339–389.
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