Definition Coverage in the OBO Foundry Ontologies: The Big Picture

High quality ontologies have both textual and logical definitions for their terms. Definitions serve many purposes: good textual definitions allow for experts and non-experts alike to understand the content of an ontology and use it in the manner the authors intended; logical definitions are necessary for reasoners to verify that an ontology is consistent, and may make application of the ontology easier for users. Ideally, logical and textual definitions would convey the same information, and each can provide an accuracy check on the other [1], [2]. Producing definitions is difficult and time-consuming. Thus, despite the best efforts of ontology developers and the existence of a number of tools and methods to populate ontologies with definitions, it is not uncommon to see missing textual or logical definitions, if not both. This is also the case in the Open Biomedical Ontologies (OBO) Foundry [3] ontologies. The OBO Foundry contains 9 ‘core’ ontologies and 128 non-core ontologies.1 These ontologies are developed in a coordinated way according to a set of shared principles.2 One of the OBO Foundry principles is about definitions: member ontologies should have “textual definitions ... for a substantial and representative fraction [of terms], plus equivalent formal definitions (for at least a substantial number of terms).”3 The statement of this principle is rather vague and elicits an obvious question: How much is ‘substantial’? We examine the coverage of textual and logical definitions throughout the OBO Foundry ontologies. In particular, we aim to determine: (1) if the prevalence of definitions is different between the core and non-core ontologies; (2) if there are more textual than logical definitions; (3) if the size of ontologies has an effect on definitional coverage. To conclude, we discuss ways of quantifying the notion of ‘substantial’ definition coverage to determine to what extent the principle of having textual and logical definitions for a substantial number of terms is upheld.