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Discourse Structure and Anaphoric Accessibility

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  1. Discourse Structure and Anaphoric Accessibility Massimo Poesio and Barbara Di Eugenio with help from Gerard Keohane

  2. Content • Empirical Investigations of Discourse Structure • Grosz and Sidner’s theory of the Global Focus • Relational Discourse Analysis • How we used RDA to study G&S • Results • Discussion Information Structure and Discourse Structure

  3. Empirical Investigations of Discourse Structure: A new opportunity • Original proposals concerning effect of discourse structure on accessibility (Reichman, 1985; Fox, 1987; Grosz and Sidner, 1986) based on unsystematic analysis of data • These days we know more about reliable studies of discourse phenomena (Passonneau and Litman, 1993; Carletta et al, 1997) • These new resources already used to propose new theories of anaphora and discourse structure such as Veins Theory (Cristea, Ide, Marcu, et al, 1998, 1999, 2000) • The goal of this project: use a reliably annotated corpus (the Sherlock corpus from the University of Pittsburgh, Moser and Moore, 1996; Di Eugenio et al, 1997) to study claims of G&S Information Structure and Discourse Structure

  4. Grosz and Sidner’s Theory of the Global Focus • The structure of a discourse is determined by the intentions utterances are meant to convey (DISCOURSE SEGMENT PURPOSES) • INTENTIONAL STRUCTURE: DOMINANCE and SAT-PRECEDES relations between DSPs • ATTENTIONAL STRUCTURE: a stack of FOCUS SPACES • Focus spaces on the stack contain accessible discourse entities • Presence on the stack reflects intentional structure • The problem: how to identify DSPs in a discourse Information Structure and Discourse Structure

  5. Relational Discourse Analysis (RDA) • Moore and Pollack, 1992; Moser and Moore, 1996 • Combines ideas from RST and Grosz and Sidner’s theory • From Grosz and Sidner: discourse structure is determined by intentional structure • RDA-SEGMENT: a segment expressing an intentional relation • From RST: segments have internal structure • CORE (cfr. NUCLEUS) • CONTRIBUTOR (cfr. SATELLITE) • Both INTENTIONAL and INFORMATIONAL relations • A fixed number of intentional relations • Has been proven to be usable for reliable analysis Information Structure and Discourse Structure

  6. RDA Analysis of an excerpt from a tutorial • 1.1 Before troubleshooting inside the text station, • 1.2 It’s always best to eliminate both the UUT and the TP • 2.1 Since the test package is moved frequently • 2.2 It is prone to damage • 3.1 Also, testing the test package is much easier and faster • 3.2 than opening up test station drawers. CONVINCE CONVINCE ENABLE Prescribed-act: Wrong-act Cause:effect step1:step2 1.1 1.2 2.1 2.2 3.1 3.2 Information Structure and Discourse Structure

  7. Moser and Moore: mapping between RST relations and G&S • Basic principles: • Every DSP must be associated with a core • Constituents of the RDA structure that do not include cores – such as clusters – do not introduce DSPs • Consequences for attentional state: • A new focus space only pushed when a segment is open • Information relations do not affect the attentional state Information Structure and Discourse Structure

  8. Mapping RDA into Attentional State • 1.1 Before troubleshooting inside the text station, • 1.2 It’s always best to eliminate both the UUT and the TP • 2.1 Since the test package is moved frequently • 2.2 It is prone to damage • 3.1 Also, testing the test package is much easier and faster • 3.2 than opening up test station drawers. DSP1 CONVINCE CONVINCE DSP 2 ENABLE Prescribed-act: Wrong-act Cause:effect step1:step2 1.1 1.2 2.1 2.2 3.1 3.2 Information Structure and Discourse Structure

  9. Using an RDA-annotated corpus to study anaphoric accessibility • The data: the SHERLOCK corpus, already annotated according to RDA instructions (Moser, 1996) • Added anaphoric annotation according to GNOME instructions (Poesio, 2000) derived from MATE scheme (Poesio Bruneseaux and Romary, 1999) • Use RDA analysis to drive focus space construction • Measure: • Accessibility • Perplexity Information Structure and Discourse Structure

  10. The Data: the SHERLOCK corpus • 17 tutorial dialogues collected within the Sherlock project (Lesgold et al, 1992) • Students solve electronic troubleshooting problem • 313 turns, 1333 clauses • RDA annotation: Moser and Moore, 1996 • Reliability verified at different levels • Intentional relations: CONCEDE, CONVINCE, ENABLE, JOINT Information Structure and Discourse Structure

  11. An example of Sherlock dialogue • STUDENT: • 1.1 Why isn't measurement signal path green during good test • readings (steps)? • TUTOR: • 2.1 For each step that passed, • 2.2 you know the measurement path is good. • 2.3 You also know that one of the measurement paths is bad. • 2.4 Showing the UUT, Test Package, and measurement section as • unknown is correct • 2.5 because, you know when you get your fail that • something was wrong, • 2.6 but you didn't know exactly what. • 2.7 The DMM is green • 2.8 because it has been working all along. • 2.9 The stimulus section is green • 2.10 because it was not used • 2.11 and is assumed to be good. Information Structure and Discourse Structure

  12. Anaphoric Annotation • The GNOME scheme (Poesio, 2000) • Mark up all NPs as NE element, with a variety of attributes • About 3000 NEs • Use separate ANTE element to mark up anaphoric relations (including bridges) • In this annotation: only direct anaphoric relations • (About 1500 total) Information Structure and Discourse Structure

  13. Evaluation • A PERL script simulates focus space construction and computes accessibility and perplexity • Accessibility: whether antecedent is in focus stack • Perplexity: Sum 1/d(xi ) m(xi) (where m(xi) = 1 if xi matches anaphor, 0 otherwise) • Parameters for focus space construction: • PUSHING: • Whenever relation is encountered (either informational or intentional) • Only when intentional • POPPING: • As soon as associated constituent is completed • Immediate popping of contributors, delayed popping of cores • Delayed popping of contributors Information Structure and Discourse Structure

  14. Evaluation I: Intentional vs Informational Accessibility: Perplexity: All = 0.83, Intentional = 1.23 Information Structure and Discourse Structure

  15. Complications ENABLE • 24.13a Since S52 puts a return (0 VDC) on it’s outputs • 24.13b when they are active, • 24.14 the inactive state must be some other voltage. • 24.15 So even though you may not know what the “other” voltage is, • 24.16 You can test to ensure that • 24.17a the active pins are 0 VDC • 24.17b and all the inactive pins are not 0 VDC. DSP 1 CONCEDE ENABLE 24.14 24.15 24.16 Effect:cause 24.13a 24.13b Contrast1: contrast2 24.17a 24.17b Information Structure and Discourse Structure

  16. Complications ENABLE • 24.13a Since S52 puts a return (0 VDC) on it’s outputs • 24.13b when they are active, • 24.14 the inactive state must be some other voltage. • 24.15 So even though you may not know what the “other” voltage is, • 24.16 You can test to ensure that • 24.17a the active pins are 0 VDC • 24.17b and all the inactive pins are not 0 VDC. DSP 1 CONCEDE ENABLE 24.14 24.15 24.16 Effect:cause 24.13a 24.13b Contrast1: contrast2 24.17a 24.17b Information Structure and Discourse Structure

  17. Evaluation II: Delayed Popping Accessibility Average perplexity with immediate popping: 1.23 Delayed popping of cores: 1.3 Delayed popping of contributors: 1.33 Perplexity Information Structure and Discourse Structure

  18. Discussion • Accessibility: • Intentional vs. informational distinction makes sense • Cfr. Fox • Want to keep contributors as well as cores on stack • cfr. Veins Theory • An evaluation of Grosz and Sidner’s framework: • The most direct implementation makes quite a few discourse entities unaccessible • Difficult to interpret more complex operations in terms of intentional structure • Alternative: a cache model (cfr. Guindon 1985, Walker 1996, 1998) • Version 1 (conservative): cache of focus spaces • Version 2: cache of forward looking centers Information Structure and Discourse Structure

  19. Cache-based global focus: a conservative proposal • Cache elements are FOCUS SPACES • Cache elements are RANKED: Current focus space < other constituents of same segment < dominating segments < focus spaces of contributors to closed spaces(Cfr. Reichman 85) • Search algorithm: follow ranking • Cache replacement algorithm: • Opening RDA segment: open new focus space, replace lowest-ranked element of cache, assign it highest rank • Closing RDA segment: Assign lowest rank to embedded contributors Information Structure and Discourse Structure