Introduction to Computational Linguistics Eleni Miltsakaki AUTH Fall 2005-Lecture 8

What’s the plan for today? Discourse models Rhetorical Structure Theory http://www.sfu.ca/rst/ Next time: The DLTAG approach

What is RST? A descriptive theory of discourse organization, characterizing text mostly in terms of relations that hold between parts of text.

History RST was developed as part of a project on computer-based generation of text by Bill Mann, Sandy Thompson and Christian Matthiessen RST is based on studies of carefully written text of a variety of sources RST is intended to describe texts (not processes of producing or understanding them) RST gives an account of coherence in text

Elements of RST Relations Schemas Schema applications Structures

Relations Relations hold between two non-overlapping text spans Nuclear:Satellite (denoted by N and S) Multi-nuclear relations

Example

RST tree

Definition of relations Constraints on nucleus Constraints on satellite Constraints on the combination of nucleus and satellite The effect

RST schemas Schemas define the structural constituency arrangement of text.

RST schema applications Unordered spans: the schemas do not constrain the order of nucleus or satellites in the text span in which the schema is applied Optional relations: for multi-relations schemas, all individual relations are optional, but at least one if the relations must hold Repeated relations: a relation that is part of a schema can be applied any number of times in the application of that schema

Basic RST relations

Evidence Relation name: EVIDENCE Constraints on N: R might not believe N to a degree satisfactory to W(riter) Constraints on S: The reader believes S or will find it credible Constraints on the N+S combination: R’s comprehending S increases R’s belief of N The effect: R’s belief of N is increased Locus of the effect: N

Example The program as published for calendar year 1980 really works. In only a few minutes, I entered all the figures from my 1980 tax return And got a result which agreed with my hand calculations to the penny. 2-3 EVIDENCE for 1

Justify Relation name: JUSTIFY Constraints on N: none Constraints on S: none Constraints on N+S combination: R’s comprehending S increases R’s readiness to accept W’s right to present N The effect: R’s readiness to accept W’s right to present N is increased Locus of the effect: N

Antithesis Relation name: ANTITHESIS Constraints on N: W has positive regard for the situation presented in N Constraints on S: none Constraints on N+S combination: the situation presented in N and S are in contrast. Because of the incompatibility that arises from contrast, one cannot have positive regard for both situations presented in N and S; comprehending S and the incompatibility between the situations presented in N and S increases R’s positive regard for the situation presented in N The effect: R’s positive regard for N is increased Locus of effect: N

Concession Relation name: CONCESSION Constraints on N: W has positive regard for the situation presented in N Constraints on S: W is not claiming that the situation presented in S doesn’t hold Constraints on the N+S combination: W acknowledges a potential or apparent incompatibility between the situations presented in N and S; recognizing the incompatibility increases R’s positive regard for the situation presented in N The effect: R’s positive regard for the situation presented in N is increased Locus of effect: N and S

Example Concern that this material is harmful to health or the environment may be misplaced. Although it is toxic to certain animals, Evidence is lacking that it has any serious long-term effect on human beings. 2 CONCESSION to 3 2-3 ELABORATION to 1

Span order

Distinctions among relations Subject matter (semantic) Two parts of the text are understood as causally related in the subject matter E.g. VOLITIONAL CAUSE Presentational (pragmatic) Facilitate presentation process E.g. JUSTIFY

What is nuclearity? Relations are mostly asymmetric E.g. If A is evidence for B, then B is not evidence for A Diagnostics for nuclearity One member is independent of the other but not vice versa One member is more suitable for substitution that the other. An EVIDENCE satellite can be replaced by entirely different evidence One member is more essential to the writer’s purpse than the other

RST annotated corpus Released via LDC (Language Data Consortium) www.ldc.upenn.edu Information, samples of the corpus plus the RST annotation tool available at www.isi.edu/~marcu/discourse

RST-based discourse parsing “An unsupervised approach to recognizing discourse relations” (2002) by D. Marcu and A. Echihabi “The rhetorical parsing of unrestricted texts: A surface-based approach” (2000) by D. Marcu