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)
syntactic subsumption
semantic subsumption T:
TE is a Graph
Matching problem! 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. What’s 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 (fc:ChCt bijective)
Step 2
Dependency matching (fd:DhDt bijective)
Step 3
Define MCSS using fc and fd
Step 4
Evaluate Similarity Measure on MCSS

10. Constituent Similarity
Degree of similarity
where t h
Parameter Box a

11. Dependency Similarity
Degree of Similarity AL
Parameter Box a

12. Textual Entailment Measure
Finally....
textual entailment holds if >t
constituents
dependencies
Parameter Box
a,d,t

13. Some more details Syntactic Transformation Other phenomena
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 (Basili&Zanzotto, 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:
a=0.85
g=0.85
d=0.5

19. Preliminary analysis (SVM-based system)
Test Bed: dev1+dev2
Test Method: 3-fold cross validation repeated 10 times
winning horse!

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.