12.7 Designing the Properties 303
As more was learned about them, the taxonomy continued to change, but the
disjointness condition was maintained.
Ontology languages differ from one another with respect to how disjoint-
ness is specified and whether it is implicitly assumed. In some ontology
languages, subclasses are necessarily disjoint, unless one specifies otherwise.
Other ontology languages presume that subclasses may overlap unless one
specifies that they are disjoint. XML DTDs do not have a mechanism for al-
lowing a particular element to belong to more than one type of element. Each
particular element has exactly one tag. Thus XML DTDs do not allow any
overlap among element types. By contrast, RDF and OWL allow instances to
belong to more than one class, as long as the classes have not been explicitly
specified to be disjoint (which can be specified in OWL, but not in RDF).
Summary
- XML hierarchies are concerned with the structure of the document.
- RDF and OWL hierarchies are concerned with the subclass relationships.
- Concept hierarchies can be developed in several ways:
- From the most general to the most specific (top-down)
- From the most specific to the most general (bottom-up)
- Starting at an intermediate, basic level (middle-out)
- Developing high quality concept hierarchies is difficult. The following
techniques have been helpful:
- Maintain a uniform structure throughout the hierarchy.
- Carefully distinguish instances from classes.
- Keep the hierarchy as simple as possible, elaborating concepts only
when necessary.
- Specify whether or not the hierarchy is strict (nonoverlapping).
12.7 Designing the Properties
One can think of the class hierarchy as being the skeleton of the ontology. It
forms the structure on which the rest of the ontology is based. The properties
form the rest of the ontology, and are analogous to the rest of the organism:
