Debugging Unsatisfiable Classes in OWL Ontologies

As an increasingly large number of OWL ontologies become available on the Semantic Web and the descriptions in the ontologies become more complicated, finding the cause of errors becomes an extremely hard task even for experts. Existing ontology development environments provide some limited support, in conjunction with a reasoner, for reporting errors in OWL ontologies. Typically these are restricted to the mere detection of, for example, unsatisfiable concepts. However, the diagnosis and resolution of the bug is not supported at all. For example, no explanation is given as to why the error occurs (e.g., by pinpointing the root clash, or axioms in the ontology responsible for the clash) or how dependencies between classes cause the error to propagate (i.e., by distinguishing root from derived unsatisfiable classes). In the former case, information from the internals of a description logic tableaux reasoner can be extracted and presented to the user (glass box approach); while in the latter case, the reasoner can be used as an oracle for a certain set of questions and the asserted structure of the ontology can be used to help isolate the source of the problems (black box approach). Based on the two approaches, we have integrated a number of debugging cues generated from our reasoner, Pellet, in our hypertextual ontology development environment, Swoop. A conducted usability evaluation demonstrates that these debugging cues significantly improve the OWL debugging experience, and point the way to more general improvements in the presentation of an ontology to users.