“This week’s question about real-world use of RDF metadata: is anybody using RDF Schema for the sake of RDF Schema, or has RDFS become little more than a layer of RDF/OWL? For example, we use rdfs:domain and rdfs:range in our RDF/OWL ontologies, but have owl:Class and owl:subClassOf completely replaced the use of rdfs:Class and rdfs:SubClassOf? In other words, has RDF/OWL, as an extension of RDFS, replaced the use of RDFS by itself, or is anyone still creating and using RDF Schemas that use nothing from the owl namespace?”
We wondered about that too and looked into it for a paper, Characterizing the Semantic Web on the Web, to appear in ISWC 2006. Here’s some text from the paper. The statistics mentioned were from data from earlier this summer, so are a bit out of date. I spot checked them and found that they are not too different today.
To what degree does the current Semantic Web make use of RDFS and OWL? One simple way of addressing this question is to examine the number of Semantic Web documents (SWDs) available on the Web that use the RDFS and OWL namespaces. The OWL namespace has been declared by 113K SWDs (8%) and actually used by 108K (7%). The RDFS namespace enjoys more use, being declared by 677K (47%) and used by 538K (37%) SWDs.
What about their terms? Not surprisingly, owl:Class is the most used term from the OWL namespace with ~ 1.8M instantiations in 68K SWDs. Contrasting this with rdfs:Class, which has 327K instantiations by 8.6K SWDs, seems to suggest that OWL is being more heavily used than RDFS. However, the relationship is not so simple. When examining properties, rdf:Property has 529K immediate instantiations from 59K SWDs, considerably more than the OWL property terms owl:ObjectProperty (170K assertions in 8K SWDs) and owl:DatatypeProperty (48K assertions in 4.6K SWDs).
For RDFS and OWL properties, the most used properties is rdf:type, followed by some annotation properties such as rdfs:seeAlso and rdfs:label. Among those properties that are used as ontology constructs, owl:sameAs and rdfs:subClassOf are the most used. We also noticed significant use of two OWL equality assertions: owl:sameAs (280K assertions in 17K SWDs) and owl:equivalentClass (70K assertions in 4.3K SWDs). Their common use may be an indication of increased ontology alignment. We have found limited use of properties that require OWL DL or OWL FULL reasoning support. The most common one in our dataset was owl:unionOf which is used in only 2.5K SWDs.