inference
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<h1 class="title">
<img src="/2008/site/images/theme-web-semantics" alt="" />Inference</h1>
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<li><a href="#summary">what is inference</a>
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<li><a href="#uses">what  inferece is used for</a>
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<h2 id="summary">What is Inference?</h2>
<p>Broadly speaking, inference on the Semantic Web can be characterized by
discovering new relationships. On the Semantic Web, data is modeled as a
set of (named) relationships between resources. “Inference” means that
automatic procedures can generate new relationships based on the data
and based on some additional information in the form of a vocabulary, e.g., a
set of rules. Whether the new relationships are explicitly added to the
set of data, or are returned at query time, is an implementation
issue.</p>
<p>On the Semantic Web, the source of such extra information can be defined
via <a href="ontology">vocabularies</a> or rule sets. Both of these approaches draw
upon knowledge representation techniques. In
general, ontologies concentrate on classification methods, putting an
emphasis on defining 'classes', 'subclasses', on how individual
resources can be associated to such classes, and characterizing the
relationships among classes and their instances. Rules, on the other
hand, concentrate on defining a general mechanism on discovering and
generating new relationships based on existing ones, much like logic
programs, like Prolog, do. In the family of Semantic Web related W3C
Recommendations RDFS, OWL, or SKOS are the tools of choice to define ontologies, whereas
RIF has been developed to cover rule based approaches.</p>
<h2 id="uses">What Inference is Used For?</h2>
<p>Inference on the Semantic Web is one of the tools of choice to
improve the quality of data integration on the Web, by discovering
new relationships, automatically analysing the content
of the data, or managing knowledge on the Web in general. Inference
based techniques are also important in discovering
possible inconsistencies in the (integrated) data.</p>
<h2 id="examples">Examples</h2>
<p>A simple example may help. The data set to be considered may include the
relationship <tt>(Flipper isA Dolphin)</tt>. An ontology may declare that “every
Dolphin is also a Mammal”. That means that a Semantic Web program
understanding the notion of “X is also Y” can add
the statement <tt>(Flipper isA Mammal)</tt> to the set of
relationships, although that was not part of the original data. One can also say that the new relationship
was “discovered”. Another example is to express that fact that “if two
persons have the same name, home page, and email address, then they are
identical”. In this case, the “identity” of two resources can be
discovered via inferencing.</p>
<p>Usage and techniques of ontologies and rules largely overlap. Very
broadly speaking, ontologies optimize for taxonomic reasoning problems,
and rule based systems optimize for reasoning problems within the data.
The difference is largely a matter of style, and criteria like available
expertise, ease of adapting to existing data, tooling support, maturity
and costs, etc., should be considered as far more important when trying
to choose.</p>
<h2 id="learnmore">Learn More</h2>
<p>The Semantic Web community maintains a <a href="http://esw.w3.org/topic/SwBooks">list of books</a>
on a W3C Wiki page. Some of those books
are introductory in nature while others are conference proceedings or
textbook that address more advanced topics.
Details of <a href="http://www.w3.org/2007/11/Talks/Semantic_Web.html">recent and upcoming
Semantic Web related talks</a>, given by the <a href="http://www.w3.org/People/">W3C Staff</a>,
the <a href="http://www.w3.org/Consortium/Offices/">staff of the W3C Offices</a>,
and members of the W3C Working Groups are available separately; the slides
are usually publicly available. The
W3C also maintains a <a href="http://www.w3.org/2001/sw/sweo/public/UseCases/">collection of Semantic
Web Case Studies and Use Cases</a> that
show how Semantic Web technologies, including inference, is used in
practice. Finally, the <a href="/2001/sw/SW-FAQ">Semantic Web FAQ</a> may also be of help
in understanding the various concepts.</p>
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