1. School of Science & Technology, Hellenic Open University, 263 35 Patras, Greece
2. Department of Management Science and Technology, University of Patras, 263 34 Patras, Greece
3. Department of Applied Informatics, University of Macedonia, 546 36 Thessaloniki, Greece
4. Department of Computer Science, International Hellenic University, 654 04 Kavala, Greece