In
linguistics
Linguistics is the scientific study of human language. It is called a scientific study because it entails a comprehensive, systematic, objective, and precise analysis of all aspects of language, particularly its nature and structure. Linguis ...
, coreference, sometimes written co-reference, occurs when two or more expressions refer to the same person or thing; they have the same
referent
A referent () is a person or thing to which a name – a linguistic expression or other symbol – refers. For example, in the sentence ''Mary saw me'', the referent of the word ''Mary'' is the particular person called Mary who is being spoken of, ...
. For example, in ''Bill said Alice would arrive soon, and she did'', the words ''Alice'' and ''she'' refer to the same person.
Co-reference is often non-trivial to determine. For example, in ''Bill said he would come'', the word ''he'' may or may not refer to Bill. Determining which expressions are coreferences is an important part of analyzing or understanding the meaning, and often requires information from the context,
real-world knowledge, such as tendencies of some names to be associated with particular species ("Rover"), kinds of artifacts ("Titanic"), grammatical genders, or other properties.
Linguists commonly use indices to notate coreference, as in ''Bill
i said he
i would come''. Such expressions are said to be ''coindexed'', indicating that they should be interpreted as coreferential.
When expressions are coreferential, the first to occur is often a full or descriptive form (for example, an entire personal name, perhaps with a title and role), while later occurrences use shorter forms (for example, just a given name, surname, or pronoun). The earlier occurrence is known as the
antecedent and the other is called a
proform
In linguistics, a pro-form is a type of function word or expression that stands in for (expresses the same content as) another word, phrase, clause or sentence where the meaning is recoverable from the context. They are used either to avoid rep ...
, anaphor, or reference. However, pronouns can sometimes refer forward, as in "When she arrived home, Alice went to sleep." In such cases, the coreference is called
cataphoric rather than anaphoric.
Coreference is important for
binding phenomena in the field of syntax. The theory of binding explores the syntactic relationship that exists between coreferential expressions in sentences and texts.
Types
When exploring coreference, numerous distinctions can be made, e.g.
anaphora,
cataphora
In linguistics, cataphora (; from Greek, '' καταφορά'', ''kataphora'', "a downward motion" from '' κατά'', ''kata'', "downwards" and '' φέρω'', ''pherō'', "I carry") is the use of an expression or word that co-refers with a later, ...
, split antecedents, coreferring noun phrases, etc. Several of these more specific phenomena are illustrated here:
:;
Anaphora
::a. The music
i was so loud that it
i couldn't be enjoyed.
–The anaphor ''it'' follows the expression to which it refers (its antecedent).
::b. Our neighbors
i dislike the music. If they
i are angry, the cops will show up soon.
– The anaphor ''they'' follows the expression to which it refers (its antecedent).
:;
Cataphora
::a. If they
i are angry about the music, the neighbors
i will call the cops.
– The cataphor ''they'' precedes the expression to which it refers (its postcedent).
::b. Despite her
i difficulty, Wilma
i came to understand the point.
– The cataphor ''her'' precedes the expression to which it refers (its postcedent)
:;
Split antecedents
::a. Carol
i told Bob
i to attend the party. They
i arrived together.
– The anaphor ''they'' has a split antecedent, referring to both ''Carol'' and ''Bob''.
::b. When Carol
i helps Bob
i and Bob
i helps Carol
i, they
i can accomplish any task.
– The anaphor ''they'' has a split antecedent, referring to both ''Carol'' and ''Bob''.
:;
Coreferring noun phrases
::a. The project leader
i is refusing to help. The jerk
i thinks only of himself.
– Coreferring noun phrases, whereby the second noun phrase is a predication over the first.
::b. Some of our colleagues
1 are going to be supportive. These kinds of people
1 will earn our gratitude.
– Coreferring noun phrases, whereby the second noun phrase is a predication over the first.
Relation to bound variables
Semanticists and logicians sometimes draw a distinction between coreference and what is known as a
bound variable
In mathematics, and in other disciplines involving formal languages, including mathematical logic and computer science, a free variable is a Mathematical notation, notation (symbol) that specifies places in an expression (mathematics), expressi ...
. Bound variables occur when the antecedent to the proform is an indefinite quantified expression, e.g.
Quantified expressions such as ''every student'' and ''no student'' are not considered referential. These expressions are grammatically singular but do not pick out single referents in the discourse or real world. Thus, the antecedents to ''his'' in these examples are not properly referential, and neither is ''his''. Instead, it is considered a ''variable'' that is ''bound'' by its antecedent. Its reference varies based upon which of the students in the discourse world is thought of. The existence of bound variables is perhaps more apparent with the following example:
This sentence is ambiguous. It can mean that Jack likes his grade but everyone else dislikes Jack's grade; or that no one likes their own grade except Jack. In the first meaning, ''his'' is coreferential; in the second, it is a bound variable because its reference varies over the set of all students.
Coindex notation is commonly used for both cases. That is, when two or more expressions are coindexed, it does not signal whether one is dealing with coreference or a bound variable (or as in the last example, whether it depends on interpretation).
Coreference resolution
In
computational linguistics
Computational linguistics is an Interdisciplinarity, interdisciplinary field concerned with the computational modelling of natural language, as well as the study of appropriate computational approaches to linguistic questions. In general, comput ...
, coreference resolution is a well-studied problem in
discourse
Discourse is a generalization of the notion of a conversation to any form of communication. Discourse is a major topic in social theory, with work spanning fields such as sociology, anthropology, continental philosophy, and discourse analysis. ...
. To derive the correct interpretation of a text, or even to estimate the relative importance of various mentioned subjects, pronouns and other
referring expression
In linguistics, a referring expression (RE) is any noun phrase, or surrogate for a noun phrase, whose function in discourse is to identify some individual object. The technical terminology for ''identify'' differs a great deal from one school of ...
s 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 as probably to ''Bill''. Pronouns such as ''himself'' have much stricter constraints. As with many linguistic tasks, there is a tradeoff between
precision and recall.
Cluster
may refer to:
Science and technology Astronomy
* Cluster (spacecraft), constellation of four European Space Agency spacecraft
* Asteroid cluster, a small asteroid family
* Cluster II (spacecraft), a European Space Agency mission to study t ...
-quality metrics commonly used to evaluate coreference resolution algorithms include the
Rand index
The RAND Corporation (from the phrase "research and development") is an American nonprofit global policy think tank created in 1948 by Douglas Aircraft Company to offer research and analysis to the United States Armed Forces. It is financed ...
, the
adjusted Rand index
Adjustment may refer to:
*Adjustment (law), with several meanings
*Adjustment (psychology), the process of balancing conflicting needs
*Adjustment of observations, in mathematics, a method of solving an overdetermined system of equations
*Calibra ...
, and different
mutual information
In probability theory and information theory, the mutual information (MI) of two random variables is a measure of the mutual dependence between the two variables. More specifically, it quantifies the " amount of information" (in units such ...
-based methods.
A particular 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
Pleonasm (; , ) is redundancy in linguistic expression, such as "black darkness" or "burning fire". It is a manifestation of tautology by traditional rhetorical criteria and might be considered a fault of style. Pleonasm may also be used for em ...
uses, which do not refer to anything specific:
Pleonastic uses are not considered referential, and so are not part of coreference.
[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.]
Approaches to coreference resolution can broadly be separated into mention-pair, mention-ranking or entity-based algorithms. Mention-pair algorithms involve
binary
Binary may refer to:
Science and technology Mathematics
* Binary number, a representation of numbers using only two digits (0 and 1)
* Binary function, a function that takes two arguments
* Binary operation, a mathematical operation that t ...
decisions if a pair of two given mentions belong to the same entity. Entity-wide constraints like
gender
Gender is the range of characteristics pertaining to femininity and masculinity and differentiating between them. Depending on the context, this may include sex-based social structures (i.e. gender roles) and gender identity. Most cultures u ...
are not considered, which leads to
error propagation
In statistics, propagation of uncertainty (or propagation of error) is the effect of variables' uncertainties (or errors, more specifically random errors) on the uncertainty of a function based on them. When the variables are the values of expe ...
. 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 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
A neural network is a network or circuit of biological neurons, or, in a modern sense, an artificial neural network, composed of artificial neurons or nodes. Thus, a neural network is either a biological neural network, made up of biological ...
architectures.
See also
*
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Notes
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.
*Li, Y., P. Musilek, M. Reformat, and L. Wyard-Scott 2009
Identification of pleonastic ''it'' using the web ''Journal of Artificial Intelligence Research'' 34, 339–389.
{{Formal semantics
Syntactic relationships
Generative syntax
Syntax