Coreference
When exploring coreference, numerous distinctions can be made, e.g. anaphora, cataphora, split antecedents, coreferring noun phrases, etc. Several of these more specific phenomena are illustrated here:
Anaphora
a. The musici was so loud that iti couldn't be enjoyed. b. Our neighborsi dislike the music. If theyi are angry, the cops will show up soon. Cataphora
a. If theyi are angry about the music, the neighborsi will call the cops. b. Despite heri difficulty, Wilmai came to understand the point. Split antecedents
a. Caroli told Bobi to attend the party. Theyi arrived together. b. When Caroli helps Bobi and Bobi helps Caroli, theyi can accomplish any task. Coreferring noun phrases
a. The project leaderi is refusing to help. The jerki thinks only of himself. b. Some of our colleagues1 are going to be supportive. These kinds of people1 will earn our gratitude. Semanticists and logicians sometimes draw a distinction between coreference and what is known as a bound variable. Bound variables occur when the antecedent to the proform is an indefinite quantified expression, e.g.
a. Every studenti has received hisi grade. b. No studenti was upset with hisi grade. 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:
c. Only Jacki likes hisi grade. 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 case. 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). 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 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-quality metrics commonly used to evaluate coreference resolution algorithms include the Rand index, the adjusted Rand index, and different mutual information-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 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. 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 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.
Relation to bound variables
Such expressions are said to be "coindexed", indicating that they should be interpreted as coreferential. The earlier occurrence is known as the antecedent) and the other is called (a proform, anaphor, or reference).