View
222
Download
2
Category
Tags:
Preview:
Citation preview
Textual Entailment as Syntactic Graph Distance: a rule based and a SVM based approach
Fabio Massimo ZanzottoDipartimento Informatica Sistemistica e ComunicazioneUniversità di Milano BicoccaItaly
Maria Teresa Pazienza and Marco PennacchiottiDepartment of Computer Science, Systems and Production
University of Roma “Tor Vergata”
Classifying Textual Entailment (TE)
Two dimensionsSemantic 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
Recognizing Textual Entailment (TE)
semantic subsumption syntactic subsumption
TE is a Graph Matching problem!
T:
H:
Graph Matching (GM)
GM is used, for instance, in Image Recognition
One Problem: distortions in the input graphs!!
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
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 relationStep 3: SVM-based Approach Using a SVM to evaluate parameters of the Graph
matching measureStep 4 Preliminary analysis of the results on the
development set
Extended Dependency Graph (XDG)
C are constituents syntactic head potential semantic
governor D are dependencies
among constituents
GM on XDG: definitions
Isomorphic subsumptionif two biiective functions fc and fd exist
Subgraph isomorphic subsumptionif it exists so that
Maximal Common Subsumption Subgraph (MCSS)given and , is the MCSS if
andthen
Finding the bijective function and evaluating the measure
Step 1 Constituent matching (fc:ChCt bijective)
Step 2 Dependency matching (fd:DhDt bijective)
Step 3 Define MCSS using fc and fd
Step 4Evaluate Similarity Measure on MCSS
Constituent Similarity
Degree of similarity
where
Parameter Box
ht
Dependency Similarity
Degree of Similarity
AL
Parameter Box
Textual Entailment Measure
Finally....
textual entailment holds if >t
Parameter Boxt
constituents dependencies
Some more details
Syntactic Transformation nominalization passive form
Other phenomena be-sentences vs appositions, e.g., the
president of XYZ is ... treating the not
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
Feature Spaces
T:
H:
Feature Spaces
Percent of common tokens and lemmas
Task Structural (Graph) Features
Subgraph matching indicators
Mean number of commonly anchored dependencies within constituents
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
Preliminary analysis (Rule-based System)
Analysis of on dev1
we decided for:=0.85=0.85=0.5
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
Treshold
Prec
Preliminary analysis (SVM-based system)
Test Bed: dev1+dev2 Test Method: 3-fold cross validation repeated 10
times
winning horse!
Out from the Fairy Tale...
... 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.
Recommended