skos:related allows the representation of associative (non-hierarchical) links, such as the relationship between an event type and a category of entities that usually participate in it. Another use of skos:related is between two categories where neither is more general or more specific. Note that skos:related allows the representation of associative (non-hierarchical) links, which can also be used to represent part-whole links that are not understood as hierarchical relationships.
skos:broader, skos:narrower and skos:related reflect what, in ISO 2788/5964, was BT, NT and RT at the concept level.
However, since SKOS has a broader scope in terms of KOS types, it does not make any recommendation as precise as in ISO 2788 on what is a valid hierarchy. It is primarily up to KOS editors to ensure that the links in their schemas do not conflict with what is observed in general KOS practice, of which thesauri are only one part. Instead, SKOS focuses on separating explicitly asserted “parent-child” links (skos:broader) from the more general “ancestor-descendant” links that can be automatically inferred from them (skos:broaderTransitive).
SKOS also allows to specialize semantic relations. However, it does not propose a standard set of such specializations. Rather, these are expected to come from other standards and guidelines, such as ISO 2788 itself.
To assert an associative relationship between two concepts, skos:related can be used:
ex:birds rdf:type skos:Concept;
ex:ornithology rdf:type skos:Concept;
The skos:related property is symmetric [OWL]. From the RDF graph above, for example, it follows that ex:ornithology is the subject of a skos:related statement that has ex:birds as its object.
Note on the (non-)transitivity of skos:related: The reader should be aware that in the SKOS data model skos:related is not defined as a transitive property. A transitive skos:related could have unintended consequences, as in the following example:
ex:renaissance skos:related ex:humanism
ex:humanism skos:related ex:philosophical anthropology.
ex:philosophicalAnthropology skos:related ex:philosophyOfMind.
ex:philosophyOfMind skos:related ex:cognitiveScience.
If skos:related were transitive, ex:renaissance would then be directly related to ex:cognitiveScence. Although each individual statement makes sense, the inferred statement may not conform to what the KOS designer originally intended.
Note on mixing hierarchy with association: The transitive closure of skos:broader is disjoint from skos:related. If resources A and B are related via skos:related, there should not be a chain of skos:broader relationships from A to B. The same occurs with skos:narrower (see).
Mapping and concept schemes
SKOS provides several properties that map concepts between different concept schemas. This can be done by asserting that two concepts have similar meaning, using the skos:exactMatch and skos:closeMatch properties. Two concepts from different concept schemas can also be mapped using properties that parallel the semantic relationships introduced in Section 2.3: skos:broadMatch, skos:narrowMatch, and skos:relatedMatch.
The display and hierarchical structure
These options should be considered:
- The hierarchical visualization. This is the visualization of the structure of a thesaurus based on the relationships between broader and narrower concepts. In this type of visualization, narrower terms are usually indented under the broader term that is their parent. Each hierarchy, starting with a “parent term,” contains terms from a single facet or sub-facet, so node labels containing facet names do not appear within hierarchies, although they may appear at the top of each hierarchy. A hierarchical display may contain node labels that specify division characteristics.
- The mono-hierarchical structure. This is the hierarchical arrangement of concepts, in a thesaurus or classification scheme, in which each concept can have only one broader concept. Compare with the polyhierarchical structure. In a monohierarchical structure, each concept can only appear in one place in the hierarchy, and other broader term relationships have to be shown as related term relationships.
- The polyhedral structure. This is the hierarchical arrangement of concepts, in a thesaurus or classification scheme, in which each concept may have more than one broader concept. Compare with the monohierarchical structure. In a polyhierarchical structure, the same concept may appear in more than one place in the hierarchy. Its attributes and relationships, and specifically its scope note and its narrower, related terms, are the same wherever it appears.
- The search thesaurus. This is the vocabulary intended to facilitate searching even if it has not been used to index the documents searched. Search thesauri are designed to facilitate the choice of terms and/or the expansion of search expressions to include terms for broader, narrower, or related concepts, as well as synonyms. Optionally, a regular thesaurus can be used as a search thesaurus.