Since it appears that the creation of subproperty of went unnoticed by many, I'd like to describe an important aspect of its proper use, and how that relates to classification.

Please note that instance of (P31) and subclass of (P279) are not valid values for subproperty of (P1647) claims, as described in the P1647 documentation [1]. For example, claims like "occupation subproperty of instance of" are invalid. The reasons for this are both technical and architectural.

On the technical side, instance of, subclass of and subproperty of are intended to be straightforwardly exportable as rdf:type, rdfs:subClassOf and rdfs:subPropertyOf. As described in On the Properties of Metamodeling in OWL [2], claims that use OWL's built-in vocabulary (e.g. rdf:type) as individuals make an ontology undecidable. If an ontology is undecidable, then queries are not guaranteed to terminate. This is a big deal. Decidability is a main goal of OWL 2 DL and a requirement in the more specialized profiles OWL 2 EL, OWL 2 RL and OWL 2 QL. Most Semantic Web ontologies aim to valid be in at least OWL 2 DL. So if Wikidata aims to be easily interoperable with the rest of the Semantic Web, we should aim to be valid in OWL 2 DL, and thus not make claims of the form "P subproperty of instance of (P31)" or "P subproperty of subclass of (P279)".

Avoiding such claims is also good design. There should be one -- and preferably only one -- obvious way to specify the type of an instance. Having a multitude of domain-specific "type" subproperties would promote an anti-pattern: using instance of as a catch-all property to make any statement under the sun that makes sense when connected with the phrase "is a".

Having a single "type" property for instances also fosters another best practice in Wikidata: asserted monohierarchy [3]. In other words, there should be only one explicit normal or preferred instance of or subclass of claim per item. Having an instance of claim and a subclass of claim on an item isn't necessarily bad (it's called "punning"), but having multiple instance of claims or multiple subclass of claims on an item is a bad smell. Items can typically satisfy a huge number of instance of claims, but should generally have only one such claim made explicitly in Wikidata.

For example, Coco Chanel (Q45661) can be said to be "instance of French person", "instance of fashion designer", "instance of female", etc. Instead of such catch-all use of instance of, Wikidata moves that knowledge into properties like country of citizenship (P27), occupation (P106) and sex or gender (P21). Coco Chanel has one explicit instance of value: human (Q5) -- a class that encapsulates essential features of the subject.

Most of Wikidata follows these general principles of classification. But a few domains of knowledge remain either somewhat of a mess, or organized but idiosyncratic. Items like the one for the German municipality of Aalen [4], with 7 instance of values -- several of them redundant -- exemplify the mess. With the deletion of domain-specific "type" properties like type of administrative territorial entity (P132) [5], we are on the right track. The solution is not to make such things subproperties of instance of, but rather to delete them and use instance of for one preferred class and put other values in other properties (note -- this may require new properties!).

The same applies for subclass of.

I encourage anyone interested in stuff like subproperty of to join the discussions ongoing at https://www.wikidata.org/wiki/Wikidata:Property_proposal/Property_metadata. The Wikidata community is currently discussing how we want to handle things like domain and range properties (e.g. should we use rdfs:domain or schema:DomainIncludes?) and whether we want to have an inverse of property (or delete all inverse properties). The outcome of these discussions will shape the interface between Wikidata and the rest of the Semantic Web.

Thanks,
Eric

https://www.wikidata.org/wiki/User:Emw

1. https://www.wikidata.org/wiki/Property:P1647
2. Boris Motik (2007). On the Properties of Metamodeling in OWL. https://www.cs.ox.ac.uk/boris.motik/pubs/motik07metamodeling-journal.pdf
3. Barry Smith, Werner Ceusters (2011). Ontological realism: A methodology for coordinated evolution of scientific ontologies. Section 1.8: Asserted monohierarchies. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3104413/#S9

4. Aalen on Wikidata as of 2015-01-10. https://www.wikidata.org/w/index.php?title=Q3951&oldid=184247296#P31

5. https://www.wikidata.org/wiki/Wikidata:Requests_for_deletions/Archive/2014/Properties/1#type_of_administrative_territorial_entity_.28P132.29