Ontology competency questions

Defining competency questions when following an ontology development methodology

Introduction

A handful of existing ontology development methodologies identify competency questions as a means of scoping an ontology. This post discusses how we can define these questions at the start of our ontology projects for scope-setting purposes, to later use their basis during the assessment phase of the ontology development process.

What competency questions are…

Competency questions are user-oriented interrogatives that allow us to scope our ontology. In other words, they are questions that our users would want to gain answers for, through exploring and querying the ontology and its associated knowledge base.

For example, an answer to a competency question could be obtained in simply browsing through a taxonomy and viewing the relationships across the ontology entities, or through the ability to determine inheritance, or through other mechanisms like ontology querying, etc.

The idea of competency questions has been acknowledged in the Protégé Simple Knowledge Engineering methodology and that pursued in the Toronto Virtual Enterprise (TOVE) project.

What competency questions are not…

Competency questions are helpful to list down but are not compulsory — so, it is up to us to decide whether we want to make use of them. Furthermore, they should not be viewed as an exhaustive list of requirements against which we come up with a test plan to evaluate our ontology and its knowledge base.

Examples of competency questions and scope definition

Say we’re interested in developing an ontology of ballpoint pens. Here are a few examples of possible competency questions to help scope our Ballpoint Pen Ontology:

• What characteristics belong to a ballpoint pen?
• Which types of components are common across all ballpoint pens?
• What are the common brands of ballpoint pens?
• Is a kind of ballpoint pen a specialisation of another?
• Can the definition of common types of ballpoint pens be used as a basis to define custom ballpoint pens?

Consequently, we could say that the scope of the Ballpoint Pen Ontology should encompass details of the characteristics and components of ballpoint pens, as well as definitions of custom and common types of ballpoint pens.

Developing and documenting competency questions

Developing competency questions requires us to have an idea of what our domain of interest is, for which our ontology will be a representation of. Information gathering is a key activity to make this happen, especially if we do not have a full command of the subject matter we need to develop an ontology for.

Documenting competency questions as well our goal and scope is best handled as a document at the start of our ontology development methodology. The idea is to keep this information somewhere handy so that we’re able to refer to and reuse it easily.

Using competency questions during ontology evaluation

As well as being relevant to the goal and scope definition phase of ontology development, competency questions are also useful during ontology evaluation. These competency questions, coupled with the functional and non-functional requirements we establish, allow us to plan the testing of our ontology and knowledge base in a more holistic way.