RDFS is useful, but does not solve all the possible
requirements
Complex applications may want more possibilities:
can a program reason about some terms?
E.g.:
"if «Person» resources
«A» e «B» have the same
«foaf:email» property, then
«A» e «B» are identical"
if somebody else defines a set of terms: are they
the same?
construct classes, not just name them
restrict a property range when used for a
specific class
disjointness or equivalence of classes
etc.
There is a need to support ontologies on the
Semantic Web
What is an ontology?
Jim Hendler
A set of knowledge terms, including the
vocabulary, the semantic interconnections and
some simple rules of inference and logic for
some particular topic
Studer et al. (1998)
An ontology is a formal, explicit specification of a
shared conceptualisation.
A 'conceptualisation' refers to an abstract
model of some phenomenon in the world by having
identified the relevant concepts of that
phenomenon.
'Explicit' means that the type of concepts
used, and the constraints on their use are explicitly
defined. For example, in medical domains, the concepts
are diseases and symptoms, the relations between them
are causal and a constraint is that a disease cannot
cause itself.
'Formal' refers to the fact that the ontology
should be machine readable, which excludes natural
language.
'Shared' reflects the notion that an ontology
captures consensual knowledge, that is, it is not
private to some individual, but accepted by a group.
Many definitions, but…
consensus among the ontology community
An ontology includes:
terms explicitly defined
knowledge we can infer
An ontology aims to capture consensual
knowledge, to reuse and share across software
applications and by groups of people
Ontologies are on the Web
The Web is intrinsically distributed
Share and export knowledge
applications can use different ontologies,
or…
…same ontologies, in different languages
equivalence of, and relations among terms become
an issue
We need Web Ontology Languages
RDFS can be considered as a simple ontology
language
OWL gives a much more complex set of
possibilities
Languages should be a compromise between
rich semantics for meaningful applications
feasibility, implementability
OWL: three sublanguages
OWL Lite
supports those users primarily needing a
classification hierarchy and simple
constraints. Provides a quick migration path for
thesauri and other taxonomies. Owl Lite also has a
lower formal complexity than OWL DL
OWL DL
supports those users who want the maximum
expressiveness while retaining computational
completeness (all conclusions are guaranteed to be
computable) and decidability (all computations
will finish in finite time)
OWL Full
for users who want maximum expressiveness and
the syntactic freedom of RDF with no computational
guarantees
Scarcely probable it will be entirely supported by
software tools implementing reasoning.
Classes in OWL
In RDFS, you can subclass existing
classes… that's all
In OWL, you can construct classes from
existing ones:
enumerate its content
through intersection, union, complement
through property restrictions
To do so, OWL introduces its own Class
and Thing to differentiate the
classes from individuals
Object and Datatype Property
owl:ObjectProperty
relate objects to other objects
owl:DatatypeProperty
relate objects to datatype values (e.g.
phoneNumber, name, birthDate, etc.)
its range are typed literals
no predefined types
we can use XML Schema data types (layered
Semantic Web architecture)
Property Characterization
In OWL, one can characterize the behavior of
properties (symmetric, transitive, functional, inverse
functional…)
Property Characterization (cont.)
owl:minCardinality
owl:maxCardinality
owl:SymmetricProperty
owl:TransitiveProperty
owl:FunctionalProperty
has at most one value for each object (e.g.
birthDate, name)
owl:InverseFunctionalProperty
two different objects cannot have the same value
(e.g. isTheSocialSecurityNumberFor etc.)
Property Characterization: an example
skos:related rdf:type
owl:SymmetricProperty
From: Antoine Isaac (with Guus Schreiber): Publishing
Vocabularies on the Web. NETTAB 2007 workshop on A
Semantic Web for Bioinformatics: Goals, Tools, Systems,
Applications. Pisa, Italy, June 14, 2007.
[
Slides]
Property Characterization: another example
skos:broader owl:inverseOf skos:narrower
From: Antoine Isaac (with Guus Schreiber): Publishing
Vocabularies on the Web. NETTAB 2007 workshop on A
Semantic Web for Bioinformatics: Goals, Tools, Systems,
Applications. Pisa, Italy, June 14, 2007.
[
Slides]
An example in OWL
The statement
The painting of the Sistine Chapel was carried out by
Michelangelo Buonarroti
Abstracting from the statement
The painting of the Sistine Chapel (the subject)
is an (instance of) activity
carried out by is a predicate
Michelangelo Buonarroti is an (instance of)
Person
In OWL (conceptually)
the paintingOfSistineChapel (E7.Activity) was
carried_out_by (P14F) MichelangeloBuonarroti
(E21.Person)
(from: Fausto Giunchiglia and Ilya Zaihrayeu: LIGHTWEIGHT
ONTOLOGIES - October 2007 -
Technical Report DIT-07-071)
Thesaurus
can be represented as a graph
nodes = thesaurus terms
edges = semantic associations
Faceted thesauri
Faceted thesauri belong to the family of KOS, which has
been used by the library community in modelling for
purposes associated with information retrieval
applications. They provide a semantic
structure at a suitable granularity for the general
problem of search and retrieval. In such
applications, where a fuzzy notion of
“aboutness“ is the basis for indexing
or classifying a document, as opposed to an
assertion of fact, the lightweight semantics of
faceted thesauri and related KOS may be more suited
than the formal semantics provided by AI
ontologies, designed for precisely modelling the
objects of interest in a domain. The SKOS standard
representation, combined with other developments in
standard identifiers and service protocols, now affords
the combination of formal syntax and informal
semantics, in Semantic Web applications and online
applications generally. This offers a cost
effective approach for annotation, search and browsing
oriented applications that don't require first order
logic.
(Douglas Tudhope & Ceri Binding: Faceted Thesauri,
Axiomathes (2008) 18:211–222 DOI DOI
10.1007/s10516-008-9031-6)
From thesauri to ontologies
Thesauri are often designed aiming to more
effective retrieval, instead of formally
representing the knowledge
A thesaurus is not automatically an ontology
Beware of common errors
broader is not transitive
related is symmetric, not
transitive
Simple Knowledge Organization System (SKOS)
Goal: porting (“Webifying”) thesauri:
representing and sharing classifications, glossaries,
thesauri, etc, as developed in the “Print
World”. For example:
The system must be simple to allow for a quick port
of traditional data (done by “traditional”
people…)
This is where SKOS comes in
SKOS is a common data model for knowledge
organization systems such as thesauri, classification
schemes, subject heading systems and taxonomies
The SKOS data model views a knowledge organization
system as a concept scheme comprising a set of
concepts
The elements of the SKOS data model are
classes and properties, however, SKOS
is not a formal knowledge representation
language (hierarchies and associatons of concepts do
not have any formal semantics)
Example: Entries in a Glossary (1)
“Assertion”
“(i) Any expression which is claimed to be true.
(ii) The act of claiming something to be true.”
“Class”
“A general concept, category or classification.
Something used primarily to classify or categorize
other things.”
“Resource”
“(i) An entity; anything in the universe. (ii) As
a class name: the class of everything; the most
inclusive category possible.”
(from the RDF Semantics Glossary)
Example: Entries in a Glossary (2)
Example: Thesaurus (1)
Term
Economic cooperation
Used For
Economic co-operation
Broader terms
Economic policy
Narrower terms
Economic integration, European economic cooperation,
…
Related terms
Interdependence
Scope Note
Includes cooperative measures in banking, trade,
…
(from UK Archival Thesaurus)
Example: Thesaurus (2)
SKOS example: multilingual labels for concepts
From: Antoine Isaac (with Guus Schreiber): Publishing
Vocabularies on the Web. NETTAB 2007 workshop on A
Semantic Web for Bioinformatics: Goals, Tools, Systems,
Applications. Pisa, Italy, June 14, 2007.
[
Slides]
SKOS example: collections
From: Antoine Isaac (with Guus Schreiber): Publishing
Vocabularies on the Web. NETTAB 2007 workshop on A
Semantic Web for Bioinformatics: Goals, Tools, Systems,
Applications. Pisa, Italy, June 14, 2007.
[
Slides]
Why Having SKOS and OWL?
OWL's precision not always necessary or even
appropriate
“OWL a sledge hammer/SKOS a
nutcracker”, or “OWL a Harley/SKOS a
bike”
complement each other, can be used in combination
to optimize cost/benefit
Role of SKOS is
to bring the worlds of library classification and
Web technology together
to be simple and undemanding enough in terms of
cost and required expertise
ATHENA
WP4 SKOS Workshop
)
SKOS: “One small step for man. One giant leap
for mankind.”?