12.1 Purpose of Ontology Development 283
- Whatwill be covered by the ontology. This is also called itsscope.Aclear
definition of the scope will prevent the ontology development effort from
expanding unnecessarily. Many ontologies have already been developed.
If an existing ontology has overlapping scope, then one should consider
reusing it. When an ontology is being developed as part of a larger project,
the scope will be dependent on the project scope. - Whowill be using the ontology. If an ontology will only be used by a few
persons, possibly only one person, then its design will be very different
from an ontology that will be used by a much larger community. Indeed,
if an ontology will only be used by one person for a short time, then it is
possible to avoid writing it down explicitly. The authors of this book built
a formal ontology to help with the writing of the book. This ontology was
very useful even though it was used by only two persons. - Whenand for how long the ontology will be used. An ontology that will be
used for a few weeks will generally have a much different design than one
that is intended to be used for decades. Generally speaking, the longer an
ontology will be used, the more effort one should invest in its design. - Howthe ontology is intended to be used. An ontology intended for in-
formation retrieval may be different from one intended to be used for
scientific experimentation.
When a design choice is made, it is helpful to document the rationale for
the choice and to refer back to the original purpose of the ontology. A design
rationale should include the alternatives that were considered as well as the
reason for the choice that was made. When an ontology development project
involves a substantial amount of effort, then the statement of purpose will
take the form of a statement ofproject requirements. Such a statement can be
regarded as the contract which the developers have agreed to fulfill.
In this chapter we will use a medical chart ontology as an example of on-
tology development. Another example is developed in the exercises at the
end of the chapter. The purpose of an ontology has a significant influence
on how it should be developed. We begin by giving an informal descrip-
tion of the purpose of this ontology:A hospital would like to make its medical
chart information more easily available in its medical information system. The plan
is to develop an ontology that will be useful for the next decade. The medical chart
information will be used only by medical personnel who have permission to access
the information. The information will be used both for immediate diagnostic deci-
sions and for statistical data mining to detect long-term trends. The ontology must