  Real-world semantic or knowledge-based systems, e.g., in the biomedical
domain, can become large and complex. Tool support for the localization and
repair of faults within knowledge bases of such systems can therefore be
essential for their practical success. Correspondingly, a number of knowledge
base debugging approaches, in particular for ontology-based systems, were
proposed throughout recent years. Query-based debugging is a comparably recent
interactive approach that localizes the true cause of an observed problem by
asking knowledge engineers a series of questions. Concrete implementations of
this approach exist, such as the OntoDebug plug-in for the ontology editor
Prot\'eg\'e.
  To validate that a newly proposed method is favorable over an existing one,
researchers often rely on simulation-based comparisons. Such an evaluation
approach however has certain limitations and often cannot fully inform us about
a method's true usefulness. We therefore conducted different user studies to
assess the practical value of query-based ontology debugging. One main insight
from the studies is that the considered interactive approach is indeed more
efficient than an alternative algorithmic debugging based on test cases. We
also observed that users frequently made errors in the process, which
highlights the importance of a careful design of the queries that users need to
answer.
