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ISSUE-185 draft response

From: Antoine Isaac <aisaac@few.vu.nl>
Date: Fri, 17 Oct 2008 11:41:56 +0200
Message-ID: <48F85DE4.6010200@few.vu.nl>
To: SWD WG <public-swd-wg@w3.org>

Hi all,

Here's a draft response to Michael on [ISSUE-185], let me know what you 
think. Note *this is just a draft, not the actual response* -- I'll wait 
for feedback from the WG before replying formally to
to Michael. (Michael: if you're lurking on this list feel free to post 
your thoughts at any time.)


Dear Michael,

Thank you for your comments [1]:

5. Order in Classification Systems

Order in a classification is important, indeed critical. Order is
evident in the juxtaposition of classes, the sequence of main classes,
and the sequence of co-ordinates in a class. Broader and narrower
relationships alone cannot represent order. So, maybe parallel encoding
is necessary to make sure that the system a classification scheme tries
to present is reflected when using SKOS.

To some degree, when order is connected to hierarchy, this can be
reflected by extensions to SKOS. The DDC for example has two parallel
hierarchies, one expressed by length of notation, the other by structure
(notes, etc.). This is handled at the moment by extending skos:narrower.

skosclass:narrowerStructural rdfs:subPropertyOf skos:narrower .

skosclass:broaderStructural rdfs:subPropertyOf skos:broader ;
  owl:inverseOf skosclass:narrowerStructural .


This is an interesting issue, and not trivial to solve in RDF. Usual RDF 
statements are order-neutral, there is no other solution than using RDF 
lists for representing ordered groups. The problem is so inherent to RDF 
itself that I think it would be counter-productive for us to propose a 
solution for this at the level of property representation (something 
like "the first subclass of this class is X").

Another reason is that this use case was not clearly identified 
beforehand in the SKOS Use Case and Requirements [2]. We can relate that 
to the SKOS mission as stated in the Introduction of the SKOS Primer: 
"The aim of SKOS is not to replace original conceptual vocabularies in 
their initial context of use, but to allow them to be ported to a shared 
space, based on a simplified model, enabling wider re-use and better 
interoperability." I think we're here in a case where standardization 
concerns dictate our not offering a specific solution.

I hope you can live with that!

Note actually that there exist a way to (maybe somewhat indirectly) 
address your specific problem, namely using a second layer of SKOS 
collections to represent the order of the classes in the class 
specialization hierarchy, as the following example:

ex:class rdf:type skos:Concept .
ex:subclass1 rdf:type skos:Concept ; skos:broader ex:class .
ex:subclass2 rdf:type skos:Concept ; skos:broader ex:class .
_:b0 rdf:type skos:OrderedCollection;
    skos:prefLabel "ordered subclasses of class1"@en; # this label is 
optional of course!
    skos:memberList _:b1.
_:b1 rdf:first ex:subclass1 ;
    rdf:rest _:b2.
_:b2 rdf:first ex:subclass2 ;
    rdf:rest rdf:nil.

Of course that's not utterly convienent from a representational 
perspective. Nor can we ensure it would be adopted for all other 
classifications represented in SKOS. But at least it is exactly how 
Collections should be used in current SKOS! As collections cannot be 
included in the semantic relationship hierarchy, they have to be used 
"in parallel" to semantic links between concepts. Note finally that even 
though the solution is complex, it might not be more than the parallel 
hierarchies you're hinting at, in fact.

If you believe however that this solution is inappriopriate, or that it 
should be mentioned in some document because the issue is more common 
that what we think, we encourage you to publish a brief best practice 
note and inform the SKOS community via the mailing list. We'd be more 
than happy to set up a "SKOS community best practices" wiki page to 
collect links to such statements! Generally elements that you have 
brought in [1] could be a useful addition to the practices presented in 

Best regards,


[1] http://lists.w3.org/Archives/Public/public-swd-wg/2008Oct/0061.html
[2] http://www.w3.org/TR/SKOS-UCR
[3] http://esw.w3.org/topic/SkosDev/ClassificationPubGuide?rev=12
Received on Friday, 17 October 2008 09:42:31 UTC

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