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Concept Coordination text for SKOS Primer

From: Ed Summers <ehs@pobox.com>
Date: Tue, 19 Aug 2008 01:00:58 -0400
Message-ID: <f032cc060808182200o1af451b2nb71fd2aeb49d109b@mail.gmail.com>
To: "SWD WG" <public-swd-wg@w3.org>

I took an action at the last face to face to offer up some text
describing why ISSUE-40 Concept Coordination was postponed.

  http://www.w3.org/2008/04/22-swd-minutes.html#action02

It's been a busy summer, and a long time coming, but here it is. If
you have the energy please give it a read over to make sure that 1) I
characterize the difference between pre and post-coordination
correctly; 2) the alternatives including ex:coordinationOf,
owl:unionOf and SPARQL are sound ; and that 3) I captured the spirit
of this decision appropriately.

//Ed

4.4 Coordinating Concepts

Indexing practices involving thesauri and other KOS often include the
notion of coordination. Coordination is an activity in which concepts
from a KOS are combined together. In general there are two kinds of
coordination: pre-coordination [1] and post-coordination [2]. The key
distinction between the two hinges on when the actual coordination
occurs in relation to an information retrieval event.

Pre-coordination is done prior to information retrieval, by a KOS
maintainer, or by an indexer who is using a KOS. For example, if a
concept scheme includes a concept for "Bicycle Repair" it is
considered to be pre-coordinated concept of the concept "Bicycle" and
"Repair".  In addition it is also considered to be pre-coordination
when an indexer takes two existing concepts from a concept scheme,
like "Bicycles" and "Repairing", and combines them with a given syntax
like "Bicycles--Repairing" to index a particular document.

Post-coordination on the other hand is performed as part of an
information retrieval task. For example if a given document is indexed
with two distinct concepts "Bicycles" and "Repairing" and a user
decides to perform a search for all documents that are indexed with
"Bicycles" and "Repairing".

SKOS has some facilities for representing concept coordination. The
most basic form of pre-coordination is certainly expressible with
SKOS:

  ex:bicycleRepair a skos:Concept ;
      skos:prefLabel "Bicycle Repair"@en .

Post-coordination as an information retrieval activity also lends
itself to representation as a SPARQL query. For example, given two
distinct concepts:

  ex:bicycles a skos:Concept ;
      skos:prefLabel "Bicycles"@en .

  ex:repairing a skos:Concept ;
      skos:prefLabel "Repairing"@en .

you could construct a SPARQL query to return only the documents that
are indexed with both concepts:

  SELECT ?document
  WHERE {
    ?document dc:subject ex:bicycles .
    ?document dc:subject ex:repairing .
  }

However the SKOS vocabulary itself does not provide any mechanism for
expressing that a given concept consists of a pre-coordination of
other concepts. Of course it is perfectly feasible to extend SKOS [3]
to establish a pattern for representing coordinated concepts. For
example it has been suggested [4] that a new property such as
<code>ex:coordinationOf</code> could be established:

  ex:coordinationOf a rdf:Property ;
    rdfs:domain skos:Concept ;
    rdfs:range rdf:List .

Which could then be used in assertions such as:

  ex:bicyclesRepairing a skos:Concept ;
      ex:coordinationOf (ex:bicycles, ex:repairing) ;
      skos:prefLabel "Bicycles--Repairing"@en .

It has also been suggested that OWL itself could be used to coordinate concepts:

  ex:bicyclesRepairing a skos:Concept ;
      owl:unionOf (ex:bicycles, ex:repairing) ;
      skos:prefLabel "Bicycles--Repairing"@en.

However, established patterns for pre-coordinations of this kind have
not yet emerged in the SKOS community. <code>ex:coordinationOf</code>
(or some equivalent extension), and the ramifications of using SKOS
with OWL [5] have not been explored fully enough yet to warrant
inclusion in the SKOS Reference [6], and consequently in this Primer.
Rather than commit to a design pattern that hasn't been proven useful,
the Semantic Web Deployment Group decided to postpone the issue of
coordination [7], to allow extension patterns to organically emerge as
SKOS is deployed. The hope is that as successful patterns are
established they can be documented as a W3C Note or some equivalent.
@@ TODO include info about wiki/discussion list here or in
Specialization section? @@

[1] http://www.willpowerinfo.co.uk/glossary.htm#pre-coord
[2] http://www.willpowerinfo.co.uk/glossary.htm#post-coord
[3] http://www.w3.org/TR/skos-primer/#secskosspecialization
[4] http://isegserv.itd.rl.ac.uk/public/skos/press/dc2006/coordination.html
[5] http://www.w3.org/TR/skos-primer/#secskosowl
[6] http://www.w3.org/TR/skos-reference/
[7] http://www.w3.org/2006/07/SWD/track/issues/40
Received on Tuesday, 19 August 2008 05:01:34 UTC

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