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Nested skos:Collections vs iso-thes:superGroup/subGroup

From: Osma Suominen <osma.suominen@helsinki.fi>
Date: Mon, 18 Jan 2016 10:23:12 +0200
To: "public-esw-thes@w3.org" <public-esw-thes@w3.org>
Message-ID: <569CA0F0.1080502@helsinki.fi>
Hi all,

The ISO 25964 SKOS extensions define a iso-thes:ConceptGroup class 
(subclass of skos:Collection) and its hierarchical properties 
iso-thes:superGroup and its inverse iso-thes:subGroup. On the other 
hand, SKOS Core also allows nesting skos:Collections using the 
skos:member property, as its range is defined as the union of 
skos:Concept and skos:Collection.

A typical use case for nesting Collections or ConceptGroups is having a 
built-in hierarchical domain/theme-oriented classification for the 
concepts within a thesaurus (sometimes also called microthesauri). This 
kind of pattern exists for example in EuroVoc, the UNESCO Thesaurus, CAB 
Classified Thesaurus, GACS and probably many others.

I'm wondering when to use one or the other representation (or both).

skos:Collection and skos:member are simpler and already exist in 
original SKOS. But skos:Collections are really quite underspecified in 
original SKOS. The SKOS Primer talks mainly about array / node label use 
cases, and it doesn't even mention that skos:member can be used between 
Collections. This fact is only shown in the SKOS Reference as S32, "The 
rdfs:range of skos:member is the union of classes skos:Concept and 
skos:Collection.", without any explanation of why or when having another 
Collection as the object would be desirable.

The ISO 25964 extensions define the two subclasses of skos:Collection 
(iso-thes:ConceptGroup and iso-thes:ThesaurusArray) much better and also 
the ISO standard defines the relationships iso-thes:superGroup and 
iso-thes:subGroup, e.g. "A higher-level group of which this group is a 
member". So this representation is much better defined and documented.

So, if you had a classified thesaurus, or one with hierarchical 
microthesauri, would you use plain skos:Collections nested with 
skos:member, or would you represent the classes/microthesauri as 
iso-thes:ConceptGroups and use iso-thes:superGroup and iso-thes:subGroup 
for representing their hierarchy? Something else? Both?

I'm asking because the newest version of UNESCO Thesaurus seems to do 
both, and this is currently a bit problematic for displaying in Skosmos 
[2] since both kinds of hierarchy are shown to the user. I'd like to 
better understand which pattern to favor (if any). This also affects the 
modeling choices to be made in thesauri I'm involved in developing, for 
example YSO and GACS.


[1] http://purl.org/iso25964/skos-thes#

[2] https://github.com/NatLibFi/Skosmos/issues/433

Osma Suominen
D.Sc. (Tech), Information Systems Specialist
National Library of Finland
P.O. Box 26 (Kaikukatu 4)
Tel. +358 50 3199529
Received on Monday, 18 January 2016 08:23:41 UTC

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