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Textual Entailment as Syntactic Graph Distance: a rule based and a SVM based approach

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Isomorphic subsumption. if two biiective functions fc and fd exist. Subgraph isomorphic subsumption. if it exists so that. Maximal Common Subsumption Subgraph (MCSS) ... – PowerPoint PPT presentation

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Title: Textual Entailment as Syntactic Graph Distance: a rule based and a SVM based approach


1
Textual Entailment as Syntactic Graph Distance
a rule based and a SVM based approach
Fabio Massimo Zanzotto Dipartimento Informatica
Sistemistica e Comunicazione Università di
Milano Bicocca Italy
Maria Teresa Pazienza and Marco
Pennacchiotti Department of Computer Science,
Systems and Production University of Roma Tor
Vergata
2
Classifying Textual Entailment (TE)
  • Two dimensions
  • Semantic dimension
  • paraphrasing (i.e., synonymy)
  • strict entailment
  • Recognition dimension
  • semantic subsumption
  • America Airlines will lay off ... ? America
    Airlines will fire ...
  • syntactic subsumption
  • American Airlines began laying off hundreds of
    flight attendants on Tuesday ? American Airlines
    will fire hundreds of flight attendants
  • direct implication
  • America Airlines will fire flight attendants ?
    hundreds of flight attendents will lose their
    jobs

3
Recognizing Textual Entailment (TE)
T
H
4
Graph Matching (GM)
  • GM is used, for instance, in Image Recognition
  • One Problem distortions in the input graphs!!

5
Textual Entailment as Graph Matching (GM)
  • Known limitations
  • distortion in the input syntactic/semantic graphs
    (errors in parsing, word sense disambiguation,
    etc.)
  • matching nodes is more complex than simple label
    matching
  • syntactic transformations should be an invariant
    phenomenon (nominalization, passivization,
    argument movement, ...)
  • textual entailment relation is an asimmetric
    relation
  • Textual Entailment Measure

6
Whats next
  • Step 1
  • Definition of the syntactic representation model
    (Extended Dependency Graph, XDG)
  • Step 2 Rule-based Approach
  • Definition of the Graph Matching measure for the
    textual entailment relation
  • Step 3 SVM-based Approach
  • Using a SVM to evaluate parameters of the Graph
    matching measure
  • Step 4
  • Preliminary analysis of the results on the
    development set

7
Extended Dependency Graph (XDG)
  • C are constituents
  • syntactic head
  • potential semantic governor
  • D are dependencies among constituents

8
GM on XDG definitions
  • Isomorphic subsumption
  • if two biiective functions fc and fd exist
  • Subgraph isomorphic subsumption
  • if it exists so that
  • Maximal Common Subsumption Subgraph (MCSS)
  • given and , is the
    MCSS if
  • and
  • then

9
Finding the bijective function and evaluating the
measure
  • Step 1
  • Constituent matching (fcCh?Ct bijective)
  • Step 2
  • Dependency matching (fdDh?Dt bijective)
  • Step 3
  • Define MCSS using fc and fd
  • Step 4
  • Evaluate Similarity Measure on MCSS

10
Constituent Similarity
  • Degree of similarity
  • where

h
t
Parameter Box a
11
Dependency Similarity
  • Degree of Similarity

Parameter Box a
12
Textual Entailment Measure
  • Finally....
  • textual entailment holds if
    gtt

Parameter Box a,d,t
13
Some more details
  • Syntactic Transformation
  • nominalization
  • passive form
  • Other phenomena
  • be-sentences vs appositions, e.g., the president
    of XYZ is ...
  • treating the not

14
Estimating Parameters with SVM
  • Main idea divide the Graph Matching measure in
    many subparts
  • Assumptions
  • The hypothesis H is a simple S-V-O sentence
  • SVM must learn parameters and thresholds
  • A possibility
  • Feature space divided in three parts
  • Subject Related Features
  • Main Verb Related Features
  • Object Related Features

15
Feature Spaces
T
H
16
Feature Spaces
  • Percent of common tokens and lemmas
  • Task
  • Structural (Graph) Features
  • Subgraph matching indicators
  • Mean number of commonly anchored dependencies
    within constituents

17
Used Resources
  • Chaos A modular and lexicalised parser for
    English and Italian (BasiliZanzotto, 1998, 2002)
    based on the extended dependency graph (XDG)
    formalism
  • WordNet
  • SVMlight

18
Preliminary analysis (Rule-based System)
  • Analysis of a on dev1
  • we decided for
  • a0.85
  • g0.85
  • d0.5

19
Preliminary analysis (SVM-based system)
  • Test Bed dev1dev2
  • Test Method 3-fold cross validation repeated 10
    times

20
Out from the Fairy Tale...
21
... and back to real life!!!!
Comdex -- once among the world's largest trade
shows, the launching pad for new computer and
software products, and a Las Vegas fixture for 20
years -- has been canceled for this year.
Los Vegas hosted the Comdex trade show for 20
years.
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