Discourse Annotation: Discourse Connectives and Discourse Relations

Discourse Annotation: Discourse Connectives and Discourse Relations

• Aravind Joshi and Rashmi Prasad
• University of Pennsylvania
• Bonnie Webber
• University of Edinburgh
• COLING/ACL 2006 Tutorial
• Sydney, July 16, 2006

Outline

• PART I
• Introduction
• Defining discourse relations
• Different approaches and their annotation
• Summary
• Discussion and Questions
• PART II
• Presentation of PDTB
• Experiments with PDTB
• Demo
• Final Discussion and Questions

Introduction

• Overall Motivation
• Richly annotated discourse corpora can facilitate theoretical advances
• as well as contribute to language technology.
• Specific Goals
• Discuss issues related to describing and annotating discourse relations.
• Describe briefly some specific approaches, which involve reasonably large corpora, highlighting the similarities and differences and how this shapes the resulting annotations.
• Describe in detail the predominantly lexicalized approach to discourse relation annotation in the Penn Discourse Treebank (PDTB) – partly released in April 2006, final release, April 2007– and illustrate some of its uses.
•  Encourage you to provide feedback and USE the PDTB!

What is a discourse relation?

• The meaning and coherence of a discourse results partly from how its constituents relate to each other.
• Reference relations
• Discourse relations
• Informational discourse relations convey relations that hold in the subject matter.
• Intentional discourse relations specify how intended discourse effects relate to each other.
• [Moore & Pollack, 1992] argue that discourse analysis requires both types.
• This tutorial focuses on the former – informational or semantic relations (e.g, CONTRAST, CAUSE, CONDITIONAL, TEMPORAL, etc.) between abstract entities of appropriate sorts (e.g., facts, beliefs, eventualities, etc.), commonly called Abstract Objects (AOs) [Asher, 1993].

• Reference Relations

• Discourse Coherence

• Discourse Relations

• Informational

• Intentional

Why Discourse Relations?

• Discourse relations provide a level of description that is
• theoretically interesting, linking sentences (clauses) and discourse;
• identifiable more or less reliably on a sufficiently large scale;
• capable of supporting a level of inference potentially relevant to many NLP applications.

How are Discourse Relations declared?

• Broadly, there are two ways of specifying discourse relations:
• Abstract specification
• Relations between two given Abstract Objects are always inferred, and declared by choosing from a pre-defined set of abstract categories.
• Lexical elements can serve as partial, ambiguous evidence for inference.
• Lexically grounded
• Relations can be grounded in lexical elements.
• Where lexical elements are absent, relations may be inferred.

Where are Discourse Relations declared?

• Similarly, there are two types of triggers for discourse relations considered by researchers:
• Structure
• Discourse relations hold primarily between adjacent components with respect to some notion of structure.
• Lexical Elements and Structure
• Lexically-triggered discourse relations can relate the Abstract Object interpretations of non-adjacent as well as adjacent components.
• Discourse relations can be triggered by structure underlying adjacency, i.e., between adjacent components unrelated by lexical elements.

Triggering Discourse Relations

• Lexical Elements
• Cohesion in Discourse (Halliday & Hasan)
• Structure
• Rhetorical Structure Theory (Mann & Thompson)
• Linguistic Discourse Model (Polanyi and colleagues)
• Discourse GraphBank (Wolf & Gibson)
• Lexical Elements and Structure
• Discourse Lexicalized TAG (Webber, Joshi, Stone, Knott)
• Different triggers encourage different annotation schemes.

Halliday and Hasan (1976)

• H&H; associate discourse relations with conjunctive elements:
• Coordinating and subordinating conjunctions
• Conjunctive adjuncts (aka discourse adjuncts), including
• Adverbs such as but, so, next, accordingly, actually, instead, etc.
• Prepositional phrases (PPs) such as as a result, in addition, etc.
• PPs with that or other referential item such as in addition to that, in spite of that, in that case, etc.
• Each such element conveys a cohesive relation between
• its matrix sentence and
• a presupposed predication from the surrounding discourse

Halliday and Hasan (1976)

• H&H; use presupposition to mean that a discourse element cannot
• be effectively decoded except by recourse to another element
• To help resolve reference
• To help identify sense
• To help recover missing (ellipsed) material
• On a level site you can provide a cross pitch to the entire slab by raising one side of the form, but for a 20-foot-wide drive this results in an awkward 5-inch slant. Instead, make the drive higher at the center.
• Here instead cannot be effectively decoded without reference to
• the presupposed predication: raising one side of the form
•  Instead of raising one side of the form, make the drive higher at the center.