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SubClassOf vs EquivalentTo

Prerequisites

This lesson assumes you have basic knowledge wrt ontologies and OWL as explained in:

SubClassOf

In this section we explain the semantics of SubClassOf, give an example of using SubClassOf and provide guidance for when not to use SubClassOf.

Semantics

If we have 

Class: C
  SubClassOf: D
Class: D

the semantics of it is given by the following Venn diagram:

Thus, the semantics is given by the subset relationship, stating the C is a subset of D. This means every individual of C is necessarily an individual of D, but not every individual of D is necessarily an individual of C.

A concrete example

Class: Dog
  SubClassOf: Pet
Class: Pet

which as a Venn diagram will look as follows:

Guidance

There are at least 2 scenarios which at first glance may seem like C SubClassOf D holds, but it does not hold, or using C EquivalentTo D may be a better option.

  1. This is typically where C has many individuals that are in D, but there is at least 1 individual of C that is not in D. The following Venn diagram is an example. Thus, to check whether you may be dealing with this scenario, you can ask the following question: Is there any individual in C that is not in D? If 'yes', you are dealing with this scanario and you should not be using C SubClassOf D.

  1. When you have determined that (1) does not hold, you may deal with the scenario where not only is every individual of C in D, but also every individual in D is in C. This means C and D are equivalent. In the case you rather want to make use of EquivalentTo.

EquivalentTo

Semantics

If we have

Class: C
    EquivalentTo: D
Class: D

this means the sets C and D fit perfectly on each other, as shown in the next Venn diagram:

Note that C EquivalentTo D is shorthand for 

Class: C
    SubClassOf: D
Class: D
    SubClassOf: C

though, in general it is better to use EquivalentTo rather than the 2 SubClassOf axioms when C and D are equivalent.

A concrete example

We all probably think of humans and persons as the exact same set of individuals.

Class: Person
    EquivalentTo: Human
Class: Human

and as a Venn diagram:

Guidance

When do you not want to use EquivalentTo?

  1. When there is an individual of C that is not in D.

  1. When there is an individual of D that is not in C.