Elias C. Stavropoulos

A transversal hypergraph approach for the frequent itemset hiding problem

We propose a methodology for hiding all sensitive frequent itemsets in a transaction database. Our methodology relies on a novel technique that enumerates the minimal transversals of a hypergraph in order to induce the ideal border between frequent and sensitive itemsets. The ideal border is then utilized to formulate an integer linear program (ILP) that answers whether a feasible sanitized database that attains the ideal border, exists. The solution of the program identifies the set of transactions that need to be modified (sanitized) so that the hiding can be achieved with the maximum accuracy. If no solution exists, we modify the ILP by relaxing the constraints needed to be satisfied so that the sanitized database preserves the privacy with guarantee but with minimum effect in data quality. Experimental evaluation of the proposed approach on a number of real datasets has shown that the produced sanitized databases exhibit higher accuracy when compared with the solutions of other well-known approaches.

A blended learning course for playfully teaching programming concepts to school teachers

In this paper we report our experiences from a University outreach program with primary and secondary education teachers of various specialties. Our goal was to improve the coding abilities of teachers through Scratch activities. The participants can in turn teach their students, multiplying that way the benefitted population. To increase the participation and the completion percentage, the activities are designed as a course in Moodle realized in discrete runs with manageable groups, in a blended learning approach. The educational material was a combination of learning objects with specific objectives, video material and try-out activities. The course has been completed by 559 teachers from various Greek districts, mainly of regional areas, with a high completion rate of 65%. The participants found the experience highly satisfying, interesting and agreed that they had been supported effectively throughout the process. In the paper we describe the rational of our approach, the design and implementation phases of the project, the outcomes and the main findings of the evaluation of the user opinions.