# Coreference Resolution

Source: https://www.potatoannotator.com/docs/guides/coreference-resolution

**Coreference resolution is the task of grouping all the mentions in a text that refer to the same thing. "Marie Curie … she … the physicist" is one chain pointing at one person.** It turns scattered mentions into entities, which is essential for summarization, question answering, and knowledge extraction.

See [Coreference](https://en.wikipedia.org/wiki/Coreference) for background.

## What annotators do

1. Mark each mention (a name, a pronoun, or a noun phrase) as a span.
2. Group mentions that refer to the same entity into a **chain**.
3. Repeat for every distinct entity in the passage.

The output is a set of chains, each a list of spans that co-refer. Chains can cross sentence boundaries, which is what makes the task harder than plain [span annotation](/docs/guides/span-annotation).

## Setting it up in Potato

Potato has a coreference annotation type that lets annotators mark mentions and link them into chains. The [coreference showcase](/showcase/coreference) is a ready-to-run example.

```yaml
annotation_schemes:
  - annotation_type: span
    name: mentions
    description: "Mark every mention (names, pronouns, noun phrases), then group mentions that refer to the same entity into a chain."
    labels: [Entity]
    allow_overlapping: true
```

Allow overlapping spans, because mentions frequently nest ("[[his] mother]").

## Common pitfalls

- **Singletons.** Decide whether to mark entities mentioned only once. It affects your counts and metrics.
- **Generic vs. specific.** "Doctors recommend rest", is "doctors" an entity to track? Write a rule.
- **Split antecedents.** "Alice and Bob … they" refers to both; decide how to represent it.

Because chains are structured, measure agreement carefully, see [Inter-Annotator Agreement](/docs/guides/inter-annotator-agreement) and adjudicate with care.

## Further reading

- [Span Annotation](/docs/guides/span-annotation)
- [Relation and Event Extraction](/docs/guides/relation-and-event-extraction)
- [Entity Linking](/docs/guides/entity-linking)