Title: Textual Entailment as Syntactic Graph Distance: a rule based and a SVM based approach
1Textual 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
2Classifying 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
3Recognizing Textual Entailment (TE)
T
H
4Graph Matching (GM)
- GM is used, for instance, in Image Recognition
- One Problem distortions in the input graphs!!
5Textual 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
6Whats 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
7Extended Dependency Graph (XDG)
- C are constituents
- syntactic head
- potential semantic governor
- D are dependencies among constituents
8GM 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
9Finding 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
10Constituent Similarity
- Degree of similarity
- where
h
t
Parameter Box a
11Dependency Similarity
Parameter Box a
12Textual Entailment Measure
- Finally....
- textual entailment holds if
gtt
Parameter Box a,d,t
13Some more details
- Syntactic Transformation
- nominalization
- passive form
- Other phenomena
- be-sentences vs appositions, e.g., the president
of XYZ is ... - treating the not
14Estimating 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
15Feature Spaces
T
H
16Feature Spaces
- Percent of common tokens and lemmas
- Task
- Structural (Graph) Features
- Subgraph matching indicators
- Mean number of commonly anchored dependencies
within constituents
17Used Resources
- Chaos A modular and lexicalised parser for
English and Italian (BasiliZanzotto, 1998, 2002)
based on the extended dependency graph (XDG)
formalism - WordNet
- SVMlight
18Preliminary analysis (Rule-based System)
- Analysis of a on dev1
- we decided for
- a0.85
- g0.85
- d0.5
19Preliminary analysis (SVM-based system)
- Test Bed dev1dev2
- Test Method 3-fold cross validation repeated 10
times
20Out 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.