1. Faculty of Economics, University of Porto, rua Dr. Roberto Frias, 4200-464 Porto, Portugal
2. Institute for Systems and Computer Engineering, Technology and Science, rua Dr. Roberto Frias, 4200-465 Porto, Portugal
3. CEOS.PP, ISCAP, Polytechnic of Porto, rua Jaime Lopes Amorim s/n, 4465-004 São Mamede de Infesta, Portugal