Download Ontology Learning from Text

Ontology Learning from Text
Methods & Tools
Definition of Ontology
‘A formal, explicit specification of a shared conceptualization’
must be
machine
understandable
not private to some individual,
but accepted by a group
types of concepts and
constraints must be clearly
defined
an abstract model of some
phenomenon in the world formed
by identifying the relevant
concepts of that phenomenon
Main elements of an ontology
wasWrittenBy
domain
range
Object property
(relation)
hasTitle
xsd:string
domain
Hierarchy of concepts
(is-a relations)
datatype property
(attribute)
range
Definition of Ontology Learning

The application of a set of methods and
techniques used for building an ontology from
scratch

Uses distributed and heterogeneous
knowledge and information sources

Allows a reduction in the time and effort
needed in the ontology development process
Ontology Learning methods from…

Unstructured sources
•

Semi-structured source
•

Involves NLP techniques, morphological and syntactic
analysis, etc.
elicit an ontology from sources that have some predefined
structure, such as XML Schema
Structured data
•
Extracting concepts and relations from knowledge contained
in structured data, such as databases
Ontology Learning ‘Layer Cake’
x, y (sufferFrom(x, y)  ill(x))
Axioms & Rules
Relations
cure (domain:Doctor, range:Disease)
is_a (Doctor, Person)
Disease:=<I, E, L>
Taxonomy (Concept hierarchies)
Concepts
{disease, illness}
Synonyms
disease, illness, hospital
Terms
Part 1  Terms Extraction
Axioms & Rules
Relations
Taxonomy (Concept hierarchies)
Concepts
Synonyms
disease, illness, hospital
Terms
Terms

Linguistic realizations of domain-specific concepts

Are the basis of the ontology learning process

Term extraction implies:
•
Linguistic processing  part-of-speech tagging,
morphological analysis, etc.
•
Statistical processing  compares the distribution of
terms between corpora
Terms Extraction: Process

Run a Part-Of-Speech (POS) tagger over the domain
corpus

Identify possible terms by constructing patterns, such
as: Adj-Noun, Noun-noun, Adj-Noun-Noun,…

Ignore Names

Identify only the relevant to the text terms by applying
statistical metrics
Linguistic Analysis: an example
Discourse
Analysis
[[It SUBJ:X1] [was PRED] still available…]
Dependency Structure
[[He SUBJ] [booked PRED] [[this] [table HEAD] NP:DOBJ]S]
(S)
Dependency Structure
[[the SPEC] [large MOD] [table HEAD] NP]
(Phrases)
[[He SUBJ] [booked PRED] [[this] [table HEAD]NP:DOBJ:X1]…]…
[[the] [large] [table] NP] [[in] [the] [corner] PP]
[work~ing V]
[table N:ARTIFACT] [table N:furniture]
[table] [2005-06-01] [John Smith]
Phrase Recognition
Morphological Analysis (stemming)
Part of Speech & Semantic Tagging
Tokenization (incl. Named-Entity Rec.)
Statistical Analysis
Statistical metrics used in terms extraction:
Term weighting (TFIDF)
N
tfidf ( w)  tf  log(
)
df ( w)
Chi-square
(obs  exp)
 
exp
Mutual Information
2
P( x, y )
mi ( x, y ) 
P( x) P( y )
TFIDF
Most popular weighting schema
N
tfidf ( w)  tf ( w)  log(
)
df ( w)
The word is more popular when it appears
several times in a document
tf(w)
The word is more important if it appears
in less documents
term frequency (number of words occurrences in a document)
df(w) document frequency (number of documents containing the word
N
number of all documents
tfidf(w) relative importance of the word in the document
Part 2  Synonyms
Axioms & Rules
Relations
Taxonomy (Concept hierarchies)
Concepts
{disease, illness}
Synonyms
Terms
Synonyms

Identification of terms that share
semantics, i.e., potentially refer to the
same concept

Methods for extracting synonyms
• Based on WordNet
• Latent Semantic Indexing (LSI)
WordNet



A lexical database for the English language
Nouns, verbs, adjectives & adverbs are grouped into sets of
synonyms (synsets)
Synsets are interlinked by means of conceptual-semantic
and lexical relations
Adapting WordNet to specific domain

Partition the set of synonymy relations defined in WordNet in
three classes:
•
•
•
Relations irrelevant in the specific domain
Relations that are relevant but incorrect in the specific
domain
Relations that are relevant and correct in the specific
domain

Remove relations from the first two classes and include
relations from the third class

Rank the rest sets according to their frequency in corpus
Latent Semantic Indexing (LSI)

LSI is a technique in NLP of analyzing relationships
between a set of documents and the terms they contain

Uses a term-document matrix which describes the
occurrences of terms in documents – Vector Space Model
Example:
doc1
database
X
computer
X
access
doc2
X
X
Part 3  Concepts
Axioms & Rules
Relations
Taxonomy (Concept hierarchies)
Disease:=<I, E, L>
Concepts
Synonyms
Terms
Concepts
Intension, Extension, Lexicon
A term may be indicate a concept if we can define its:
Intension:
(in)formal definition of the set of objects that this concept
describes
Example: a disease is an impairment of health or a condition of abnormal functioning
Extension: a set of objects that the definition of this concept
describes
Example: influenza, cancer, heart disease
Lexical realizations: the term itself and its multilingual synonyms
Example: disease, illness, maladie
Part 4  Taxonomy Induction
Axioms & Rules
Relations
is_a (Doctor, Person)
Taxonomy (Concept hierarchies)
Concepts
Synonyms
Terms
Concept Hierarchy Extraction
Basic methods used for taxonomy extraction:

With the use of WordNet

Lexico-syntactic patterns

Machine Readable Dictionaries

Co-occurrence Analysis

Linguistic-approaches
Taxonomy Extraction with WordNet

Given two terms t1 and t2, check if they stand in a
hypernym relation with regard to WordNet

Normalize the number of hypernym paths by dividing
by the number of senses of t1
isa(t1, t 2)  min(
| paths( senses(t1), senses(t 2)) |
,1)
| senses(t1) |
path: a sequence of edges connecting the two synsets
Example: - 4 different hypernym paths between synsets ‘country’ and ‘region’
- ‘country’ has 5 senses
value of isa (country, region) = 0.8
Lexico-syntactic patterns - Hearst

Aim: the acquisition of hyponym lexical relations from text

Uses a set of predefined lexico-syntactic patterns which
•
•
•
occur frequently and in many text genres
indicate the relation of interest
can be recognized with little or no pre-encoded knowledge

Principle idea: match these patterns in texts to retrieve
is_a relations

Precision with respect to WordNet: 55,45%
Lexico-syntactic patterns - Hearst
NPo such as {NP1, NP2,…, (and | or)} NPn
vehicle
‘Vehicles such as cars, trucks and bikes….’
is-a
is-a
is-a
truck
car
bike
such NP as {NP,} * { (or | and) } NP
fruit
‘Such fruits as oranges, nectarines or apples…’
is-a
is-a
is-a
orange
apple
nectarine
NP {, NP} * { , } { or | and } other NP
‘Swimming, running, or/and other activities…’
activity
is-a
running
is-a
swimming
Lexico-syntactic patterns - Hearst
NP { , } including {NP, } * { or | and } NP
‘Injuries, including broken bones, wounds and bruises…’
injury
is-a
is-a
is-a
broken bone
bruise
wound
NP { , } especially {NP, } * { or | and } NP
‘Publications, especially papers and books…’
publication
is-a
paper
is-a
book
Machine Readable Dictionaries

A method for extracting taxonomies which goes back
to the 80’s

Main idea: exploit the regularity of dictionary entries to
find a suitable hypernym for the defined word
Example:
spring “the season between winter and summer and in which
leaves and flowers appear”
is_a (spring, season)
MRDs: Exceptions

The hypernym can be preceded by an expression such as ‘a kind of’,
‘a sort of’, or ‘a type of’

The problem is solved by keeping an exception list with words such as
‘kind’, ‘sort’, ‘type‘ and taking the head of the NP following the
preposition ‘of’
Example: hornbeam: “a type of tree with a hard wood, sometimes used in hedges”
is_a (hornbeam, tree)

The word can be defined in terms of a part-of or membership relation
Example: republican : “a member of a political party advocating republicanism”
is_a (republican, political party) part_of (republican, political party)
Co-occurrence analysis

A certain term t1 is more special that a term t2, if
t2 also appears in all the documents in which t1
appears.
Document-based subsumption
Term x subsumes term y iff P(x | y) 1, where
P( x | y ) 
n ( x, y )
n( y )
n(x,y)  the number of documents in which x and y co-occur
n(y)  the number of documents that contain y
Linguistic Approaches

Modifiers typically restrict or narrow
down the meaning of the modified noun
Example:
is_a (international credit card, credit card)
Part 5  Relations
(non-taxonomic)
Axioms & Rules
cure (domain:Doctor, range:Disease)
Relations
Taxonomy (Concept hierarchies)
Concepts
Synonyms
Terms
Extracting relations & attributes

Specific relations
• Part-of
• Qualia (Formal, Constitutive, Telic, Agentive)

General relations
• Exploiting linguistic structure

Attributes
Learning attributes: Introduction

Attributes  relations with a datatype as range

Typically expressed in texts using preposition of, the verb have or
genitive constructs, e.g. ‘the color of the car’, ‘the car’s color’, ‘every
car has a color’

Values of attributes are expressed using copula constructs,
adjectives or expressions specific to the attribute in question, e.g.,
•
•
•
‘the car is red’ (copula + value)
‘the red car’ (adjective)
‘the baby weights 3 kgr’ (specific expressions)
Classification of attributes
To systematize the learning process attributes are classified according to their range
RelExt
A tool for Relation Extraction

identifies relevant triples (pairs of concepts connected by a
relation) over concepts from an existing ontology

is based on the fact that verbs express a relation between two
classes that specify the domain and range

extracts relevant verbs & their grammatical arguments and
computes corresponding relations through a statistical &
linguistic processing

was developed in the context of SmartWeb project to provide
intelligent information services in the FIFA World Cup 2006
RelExt: Linguistic processing
● Linguistic annotation
 the SCHUG system was used
 provides a multi-layer XML format for a
given text
 dependency structure, lemmatization,
POS
Corpus
Linguistic
annotation
NER &
Concept Tagging
● NER (Name Entity Recognition)
 performed to map instances of football
players to existing ontology classes
●Concept tagging
 maps synonyms for given terms to the
corresponding ontology concepts
Annotated
corpus
RelExt: Statistical Processing



Relevance Measure
•
χ2
test used to compute relevance
ranking
Coocurence measure
Relation Extraction
Relevance
Measure
Frequencies
In BNC, NZZ
Relevance Scores
Heads, Preds
Cooccurence
measure
Cooccurence
Scores
Heads <> Preds
Part 6  Axioms & Rules
x, y (sufferFrom(x, y)  ill(x)
Axioms & Rules
Relations
Taxonomy (Concept hierarchies)
Concepts
Synonyms
Terms
DIRT
Discovery of Inference Rules from Text

an unsupervised method for discovering inference rules
from text, such as
X is author of Y  X wrote Y,
X caused Y  Y is blamed on X
X manufactures Y  X’s Y factory

Is based on the assumption that:
Distributional Hypothesis
Words that occurred in the same contexts tend to be similar
DIRT: Distributional Hypothesis

Distributional Hypothesis is applied to
dependency tress

If two paths tend to link the same sets of
words, their meanings are hypothesized to be
similar
DIRT: Dependency trees
“John found a solution to the problem”
found
subj
obj
Links represent dependency relationships
Direction: from the head to the modifier
John
solution
Labels represent types of dependency relations
det
mod
Each link between two words represents a direct
semantic relationship
a
to
pcomp
Path between “John” and “problem”
problem
N:subj:V  find  V:obj:N  solution  N:to:N
meaning “X finds solution to Y”
det
the
DIRT: Paths in Dependency Trees
Connect the prepositional complement directly to the words
modified by the preposition
transformation rule
Each link between two words represent a direct semantic relationship
A path represents indirect semantic relationships between two content words
Ontology Learning Tools

Text2Onto
•
•

http://ontoware.org/projects/text2onto
OntoLT
•
•

Open source (Java)
Open source (Protégé plug-in, Java)
http://olp.dfki.de/OntoLT/OntoLT.htm
OntoGen
•
•
Open source (C++, .NET)
http://www.textmining.net
Conclusions

A detailed methodology that guides the ontology
learning process does not exist

Only general guidelines are provided

No complete correspondence between the methods
and the tools

Methods are based mainly on NLP techniques
complemented with statistical measures

Tools give only support to perform some of the steps
proposed in different approaches (except Text2Onto)
Some References…

Cimiano, P. Ontology Learning and Population from Text:
Algorithms, Evaluation and Applications. Springer, 2006

Hearst, M.A., Automatic Acquisition of Hyponyms from Large
Text Corpora. In: Proceedings of the 14th International
Conference on Computational Linguistics, pp. 539-545, 1992

Gómez-Pérez, A., & Manzano-Macho, D., An overview of
methods and tools for ontology learning from text, The
Knowledge Engineering Review, Vol. 19:3, 187-212, 2005.

P. Cimiano, J. Wenderoth, Automatically Learning Qualia
Structures from the Web. In: Proceedings of the ACL
Workshop on Deep Lexical Acquisition, pp. 28-37, 2005