Explaining Query Answers in Lightweight Ontologies

Accessing data through ontologies expressed in Description Logics (DL) is important for the realization of the Semantic Web. DL-LiteA, member of the DL-Lite family of DLs, is particularly suitable for answering Conjunctive Queries over large data sets. In order to meet usability requirements, most logic-based applications provide explanation facilities for reasoning services. This holds also for DL-LiteA, where research focused on the explanation of both TBox reasoning and, more recently, query answering. This thesis consists of a complete study on the latter problem, hence, we study both explanations for the absence and the presence of a tuple in the answers. Unfortunately, it happens that users are not always provided with the result they are expecting when querying an ontology. Then, it is important to provide them with an high-level motivation for the absence of a tuple in the results, i.e., find an explanation for a negative answer. In fact, in contrast with standard database technology, in the ontology setting this problem is not easily solvable by looking at the structure of the data stored in the data-layer and, for example, relaxing the conditions imposed in the query. The reason is that the reasoning involved for answering queries, provides an additional layer obfuscating the way the data is accessed by the system. We address the problem of explaining negative answers for (conjunctive) query answering over DL-LiteA ontologies, by adopting abductive reasoning, that is, we look for additions to the ABox that force a given tuple to be in the result. As reasoning tasks, we consider existence and recognition of an explanation, and relevance and necessity of a certain assertion for an explanation. An important aspect in explanations is to provide the user with solutions that are simple to understand and free of redundancy, hence as small as possible. To address this requirement, we study various restrictions on solutions, in particular, we focus on subset minimal and cardinality minimal ones. Besides explaining the absence of a tuple in a query answer, it is important to explain also why a given tuple is returned by the system. The presence of a tuple in the results of a query over a DL-LiteA ontology does not only depend on the data but also on the constraints expressed at the conceptual level. For this reason, it is important to provide users with insights on how conceptual information has been used to return a certain tuple in the answers. This problem has been tackled already in the literature, where an high-level procedure explaining positive answers to conjunctive queries over DL-LiteA ontologies is introduced. As now, the mentioned procedure has never been given a formal algorithmic counterpart and, therefore, it is not easily implementable in a real world system. For this reason, we close the gap between theory and practice by providing such an algorithmic solution to the problem. Also, we improve the given procedure by considering minimal explanations for positive answers. Further, this thesis contributes with a new use of this algorithm to solve the problem of explaining to domain users the inconsistency of a DL-LiteA knowledge base.