Ontology Development Methodologies – Grueninger and Fox

As you know, I am somewhat enamoured of ontologies and recently had cause to attend a meeting, where methods for ontology development and engineering were part of the discussion.

Now in software development, there are a number of methodologies, the community has converged on, such as the Capability Maturity Model (CMM), Waterfall Models and ISO standards, such as ISO 15504 and even ISO 9000.

Unfortunately this is not the case in ontological engineering where a number of different methodologies exist at the moment at the conceptual level (i.e. there is currently little to no tool support implementing these engineering approaches). So i thought I should write a series of blog posts, which will discuss some of the most common methodologies in ontological engineering in the hope that someone out there will find it useful (in the Long Tail I trust). Incidentally, you can tell I am on a train once more – I only get this sort of more fundamental high-level stuff done when travelling.

So first up is the methodology published bye Gueninger and Fox [1]. In pictorial form, the development process can be represented as follows:

grueninger-and-fox.gif

Grueninger and Fox argue, that ontologies are constructed in response to an application or a particular problem, which is to be solved and hence a ontological engineering should begin with defining these problems in order to define an ontology’s scope. This should take the form of scenarios and questions that one might wish to ask of the ontology and which it should be capable of answering. In this sense, the questions define the compentency of the ontology and Grueninger and Fox have therefore coined the term “competency question”.

In a subsequent step, the terms of the ontology are defined. This includes objects, attributes and relationships. In essence, this defines the language the ontology will use and also provides semantic constraints. This is followed by the formal specification of the ontology (in formal language).

In the last step, the competency of the ontology is formally tested. Let’s look at all of these stages in greater detail.

Motivating Scenarios

As already stated, the basic assumption here is, that ontologies are constructed in response to a particular problem or requirement that might arise within the context of an application scenario. It can be argued, that quite often, the scenarios can be expressed in the form of story problems which can be written down and which not only often provide implicit solutions, but also allow a first and informal glimpse of the semantics of objects and relations, which will subsequently be included in the formal ontology.

Informal Competency Questions

Quite often, the consequence of the preparation of story problems and motivating scenarios will be the emergence of questions, that the ontology should be able to answer. These questions can be considered to represent queries over the ontology, which, at this stage have not been formalised. As such, they define the scope and therefore compentency of the ontology. Furthermore, the competency questions not only define the scope of the ontology, they also provide a good deal of the justification for building it in the first place and furthermore provide a first informal evaluation: can all the questions that ought to be asked of an ontology be answered within the ontology’s envisaged competency or is an extension required? As Grueninger and Fox point out, “[…]competency questions do not generate ontological commitments; rather, they are used to evaluate the ontological commitments that have been made.”

Formalisation of the Ontology

Once the scope and semantics of the ontology have been defined in this way, it is time for the formalisation of the ontology, i.e. the expression and definition of the ontology’s terminology in the form of objects, attributes and relationships in formal language. A large number of ontology languages can in principle be chosen from, though most modern ontoogies are constructed in either OBO or OWL.

Formal Competency Questions

The nest step is the formalisation of the competency questions in the form of entailments or consistency questions with respect to the axioms contained in the ontology. The formalisation of competency questions is an important part of the evaluation of the constructed ontology. It is also their main distinguishing feature: differentiation of ontologies occurs through the competency questions that different ontologies can answer. It is important to realise at this stage, that formal competency questions do no axiomatize the ontology, but will be used to evaluate its completeness.

Specification of Axioms

The definition of axioms is probably the most difficult part of the ontolgy engineering process. However, it the axioms in the ontology, which both specify the definition of terms as well as constrain their interpretation. In other words, it is the axioms, that provide the semantics of the ontology terms.
As Grueninger and Fox point out, axioms are central for ontology construction: “[…]without the axioms, we cannot express the question or its solution, and with the axioms we can express the question and its solutions.” Solutions to the competency questions must be entailed by the axioms in the ontology. If inconsistencies are found or formal competency questions cannot be answered, the ontology must be changed or extended until everything is satisfactory.

Completeness

In a last step, the ontology engineer must provide the conditions under which the solutions to formally stated competency questions are considered to be complete. This provides the basis for the formulation of completeness theorems.

Thus far Grueninger and Fox and half a knackered laptop battery later. Stay tuned for the next exciting installment of……..Uschold and King….:-).

[1] M. Grueninger, M. Fox, “Methodology for the Design and Evaluation of Ontologies”, IJCAI’95, Workshop on Basic Ontological Issues in Knowledge Sharing, April 13, 1995, url = citeseer.ist.psu.edu/grninger95methodology.html

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