1. Doctoral School, Institute of Management, University of Szczecin, 71-004 Szczecin, Poland
2. Faculty of Economics, Department of Applied Mathematics in Economy, West Pomeranian University of Technology, 70-310 Szczecin, Poland
3. Department of Economics, University of Messina, 98121 Messina, Italy
4. Institute of Management, University of Szczecin, 71-004 Szczecin, Poland