1. Professor, Dept. of Production Engineering and Management, School of Engineering, Democritus Univ. of Thrace, Vas. Sofias 12, 67100 Xanthi, Greece (corresponding author).
2. Ph.D. Candidate, Dept. of Production and Management Engineering, School of Engineering, Democritus Univ. of Thrace, Vas. Sofias 12, 67100 Xanthi, Greece.