Why people care

• Classic: "text understanding"• Information extraction, information

retrieval, summarization…

What influences pronoun resolution?

• Syntax• Semantics/world knowledge

Why syntax matters

• John kicked Bill. Mary told him to go home.

• Bill was kicked by John. Mary told him to go home.

• John kicked Bill. Mary punched him.

Why syntax matters

• John kicked Bill. Mary told him to go home.

• Bill was kicked by John. Mary told him to go home.

• John kicked Bill. Mary punched him.

John

Why syntax matters

• John kicked Bill. Mary told him to go home.

• Bill was kicked by John. Mary told him to go home.

• John kicked Bill. Mary punched him.

Bill

Why syntax matters

• John kicked Bill. Mary told him to go home.

• Bill was kicked by John. Mary told him to go home.

• John kicked Bill. Mary punched him.

Bill

Why syntax matters

• John kicked Bill. Mary told him to go home.

• Bill was kicked by John. Mary told him to go home.

• John kicked Bill. Mary punched him.

Grammatical role hierarchy

Why syntax matters

• John kicked Bill. Mary told him to go home.

• Bill was kicked by John. Mary told him to go home.

• John kicked Bill. Mary punched him.

Grammatical role

parallelism

Why semantics matters

The city council denied the demonstrators a permit because they {feared|advocated} violence.

Why semantics matters

The city council denied the demonstrators a permit because they {feared|advocated} violence.

Why semantics matters

The city council denied the demonstrators a permit because they {feared|advocated} violence.

Why knowledge matters

• John hit Bill. He was severely injured.

Margaret Thatcher admires Hillary Clinton, and George

W. Bush absolutely worships her.

Why Knowledge Matters

A search-based solution

• Hobbs 1978: Resolving pronoun references

Hobbs 1978

• Assessment of difficulty of problem• Incidence of the phenomenon• A simple algorithm that has

become a baseline

Hobbs’s point

…the naïve approach is quite good. Computationally speaking, it will be a long time before a semantically based algorithm is sophisticated enough to perform as well, and these results set a very high standard for any other approach to aim for.

Hobbs’s point

Yet there is every reason to pursue a semantically based approach. The naïve algorithm does not work. Any one can think of examples where it fails. In these cases it not only fails; it gives no indication that it has failed and offers no help in finding the real antecedent.

(p. 345)

A parse tree