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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.


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.


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

  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
    ?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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