Subject indexing and SKOS
Although it does not formally belong to the defining characteristics of a KOS, the link between a concept and the resources that deal with this concept is fundamental in many KOS applications, such as indexing and document retrieval. This is even more important in the context of the semantic web, where there is a crucial need to annotate documents with conceptual units that define their subject matter.
Although the SKOS vocabulary itself does not include a mechanism for associating an arbitrary resource with a skos:Concept, implementers can draw on other vocabularies. Dublin Core, for example, provides a property dct:subject [DC]:
ex1:platypus rdf:type skos:Concept;
Note that the same resource may have several subjects and thus be implied by several dct:subject statements. These subjects may clearly come from different conceptual schemes, resulting, for example, from a distributed annotation process.
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. In general, there are two types of coordination: precoordination and postcoordination [WillpowerGlossary]. The key distinction between the two is based on when the actual coordination occurs in relation to an information retrieval event.
Precoordination is performed prior to information retrieval, by a KOS maintainer or by an indexer using a KOS; for example, if an indexer takes two existing concepts from a conceptual schema, such as “Bicycles” and “Repair,” and explicitly combines them with a particular syntax, such as “Bicycles–Repair,” to index a particular document.
Postcoordination, 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 “Repair” and a user decides to perform a search for all documents indexed with “Bicycles” and “Repair”.
Post-coordination as an information retrieval activity lends itself to an indirect representation as a SPARQL query to access RDF data [SPARQL]. For example, given two distinct concepts
ex:bicycles skos:prefLabel “Bicycles”@en.
ex:repair skos:prefLabel “Repair”@en.
a SPARQL query could be constructed to return only the documents indexed with both concepts
?document dct:subject ex:bicycles.
?document dct:subject ex:repairing.
However, the SKOS vocabulary itself does not provide any mechanism for expressing that a given concept consists of a precoordination of other concepts. Of course, it is perfectly possible to extend SKOS to establish a pattern for representing coordinated concepts.