146 research outputs found
Repairing Ontologies via Axiom Weakening.
Ontology engineering is a hard and error-prone task, in which
small changes may lead to errors, or even produce an inconsistent
ontology. As ontologies grow in size, the need for automated
methods for repairing inconsistencies while preserving
as much of the original knowledge as possible increases.
Most previous approaches to this task are based on removing
a few axioms from the ontology to regain consistency.
We propose a new method based on weakening these axioms
to make them less restrictive, employing the use of refinement
operators. We introduce the theoretical framework for
weakening DL ontologies, propose algorithms to repair ontologies
based on the framework, and provide an analysis of
the computational complexity. Through an empirical analysis
made over real-life ontologies, we show that our approach
preserves significantly more of the original knowledge of the
ontology than removing axioms
Two Approaches to Ontology Aggregation Based on Axiom Weakening
Axiom weakening is a novel technique that allows
for fine-grained repair of inconsistent ontologies.
In a multi-agent setting, integrating ontologies corresponding
to multiple agents may lead to inconsistencies.
Such inconsistencies can be resolved after
the integrated ontology has been built, or their
generation can be prevented during ontology generation.
We implement and compare these two approaches.
First, we study how to repair an inconsistent
ontology resulting from a voting-based aggregation
of views of heterogeneous agents. Second,
we prevent the generation of inconsistencies by letting
the agents engage in a turn-based rational protocol
about the axioms to be added to the integrated
ontology. We instantiate the two approaches using
real-world ontologies and compare them by measuring
the levels of satisfaction of the agents w.r.t.
the ontology obtained by the two procedures
Repairing Description Logic Ontologies by Weakening Axioms
The classical approach for repairing a Description Logic ontology O in the sense of removing an unwanted consequence α is to delete a minimal number of axioms from O such that the resulting ontology O´ does not have the consequence α. However, the complete deletion of axioms may be too rough, in the sense that it may also remove consequences that are actually wanted. To alleviate this problem, we propose a more gentle way of repair in which axioms are not necessarily deleted, but only weakened. On the one hand, we investigate general properties of this gentle repair method. On the other hand, we propose and analyze concrete approaches for weakening axioms expressed in the Description Logic EL
Reasoning in Description Logic Ontologies for Privacy Management
A rise in the number of ontologies that are integrated and distributed in numerous application systems may provide the users to access the ontologies with different privileges and purposes. In this situation, preserving confidential information from possible unauthorized disclosures becomes a critical requirement. For instance, in the clinical sciences, unauthorized disclosures of medical information do not only threaten the system but also, most importantly, the patient data. Motivated by this situation, this thesis initially investigates a privacy problem, called the identity problem, where the identity of (anonymous) objects stored in Description Logic ontologies can be revealed or not. Then, we consider this problem in the context of role-based access control to ontologies and extend it to the problem asking if the identity belongs to a set of known individuals of cardinality smaller than the number k. If it is the case that some confidential information of persons, such as their identity, their relationships or their other properties, can be deduced from an ontology, which implies that some privacy policy is not fulfilled, then one needs to repair this ontology such that the modified one complies with the policies and preserves the information from the original ontology as much as possible. The repair mechanism we provide is called gentle repair and performed via axiom weakening instead of axiom deletion which was commonly used in classical approaches of ontology repair. However, policy compliance itself is not enough if there is a possible attacker that can obtain relevant information from other sources, which together with the modified ontology still violates the privacy policies. Safety property is proposed to alleviate this issue and we investigate this in the context of privacy-preserving ontology publishing. Inference procedures to solve those privacy problems and additional investigations on the complexity of the procedures, as well as the worst-case complexity of the problems, become the main contributions of this thesis.:1. Introduction
1.1 Description Logics
1.2 Detecting Privacy Breaches in Information System
1.3 Repairing Information Systems
1.4 Privacy-Preserving Data Publishing
1.5 Outline and Contribution of the Thesis
2. Preliminaries
2.1 Description Logic ALC
2.1.1 Reasoning in ALC Ontologies
2.1.2 Relationship with First-Order Logic
2.1.3. Fragments of ALC
2.2 Description Logic EL
2.3 The Complexity of Reasoning Problems in DLs
3. The Identity Problem and Its Variants in Description Logic Ontologies
3.1 The Identity Problem
3.1.1 Description Logics with Equality Power
3.1.2 The Complexity of the Identity Problem
3.2 The View-Based Identity Problem
3.3 The k-Hiding Problem
3.3.1 Upper Bounds
3.3.2 Lower Bound
4. Repairing Description Logic Ontologies
4.1 Repairing Ontologies
4.2 Gentle Repairs
4.3 Weakening Relations
4.4 Weakening Relations for EL Axioms
4.4.1 Generalizing the Right-Hand Sides of GCIs
4.4.2 Syntactic Generalizations
4.5 Weakening Relations for ALC Axioms
4.5.1 Generalizations and Specializations in ALC w.r.t. Role Depth
4.5.2 Syntactical Generalizations and Specializations in ALC
5. Privacy-Preserving Ontology Publishing for EL Instance Stores
5.1 Formalizing Sensitive Information in EL Instance Stores
5.2 Computing Optimal Compliant Generalizations
5.3 Computing Optimal Safe^{\exists} Generalizations
5.4 Deciding Optimality^{\exists} in EL Instance Stores
5.5 Characterizing Safety^{\forall}
5.6 Optimal P-safe^{\forall} Generalizations
5.7 Characterizing Safety^{\forall\exists} and Optimality^{\forall\exists}
6. Privacy-Preserving Ontology Publishing for EL ABoxes
6.1 Logical Entailments in EL ABoxes with Anonymous Individuals
6.2 Anonymizing EL ABoxes
6.3 Formalizing Sensitive Information in EL ABoxes
6.4 Compliance and Safety for EL ABoxes
6.5 Optimal Anonymizers
7. Conclusion
7.1 Main Results
7.2 Future Work
Bibliograph
Completing and Debugging Ontologies: state of the art and challenges
As semantically-enabled applications require high-quality ontologies,
developing and maintaining ontologies that are as correct and complete as
possible is an important although difficult task in ontology engineering. A key
step is ontology debugging and completion. In general, there are two steps:
detecting defects and repairing defects. In this paper we discuss the state of
the art regarding the repairing step. We do this by formalizing the repairing
step as an abduction problem and situating the state of the art with respect to
this framework. We show that there are still many open research problems and
show opportunities for further work and advancing the field.Comment: 56 page
Towards Even More Irresistible Axiom Weakening
Axiom weakening is a technique that allows for a fine-grained repair of inconsistent ontologies. Its main advantage is that it repairs on- tologies by making axioms less restrictive rather than by deleting them, employing the use of refinement operators. In this paper, we build on pre- viously introduced axiom weakening for ALC, and make it much more irresistible by extending its definitions to deal with SROIQ, the expressive and decidable description logic underlying OWL 2 DL. We extend the definitions of refinement operator to deal with SROIQ constructs, in particular with role hierarchies, cardinality constraints and nominals, and illustrate its application. Finally, we discuss the problem of termi- nation of an iterated weakening procedure
On Expert Behaviors and Question Types for Efficient Query-Based Ontology Fault Localization
We challenge existing query-based ontology fault localization methods wrt.
assumptions they make, criteria they optimize, and interaction means they use.
We find that their efficiency depends largely on the behavior of the
interacting expert, that performed calculations can be inefficient or
imprecise, and that used optimization criteria are often not fully realistic.
As a remedy, we suggest a novel (and simpler) interaction approach which
overcomes all identified problems and, in comprehensive experiments on faulty
real-world ontologies, enables a successful fault localization while requiring
fewer expert interactions in 66 % of the cases, and always at least 80 % less
expert waiting time, compared to existing methods
Axiom Pinpointing
Axiom pinpointing refers to the task of finding the specific axioms in an
ontology which are responsible for a consequence to follow. This task has been
studied, under different names, in many research areas, leading to a
reformulation and reinvention of techniques. In this work, we present a general
overview to axiom pinpointing, providing the basic notions, different
approaches for solving it, and some variations and applications which have been
considered in the literature. This should serve as a starting point for
researchers interested in related problems, with an ample bibliography for
delving deeper into the details
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