1. a GE Control Systems , Belgium (formerly of Katholieke Universiteit, Leuven, Belgium) E-mail:
2. b Logistics Systems Dynamics Group , Cardiff Business School, Cardiff University , Colum Drive, Cardiff, CF10 3EU, Wales, UK E-mail:
3. c Department of Applied Economics , Katholieke Universiteit , Leuven, Belgium E-mail:
4. d Logistics Systems Dynamics Group , Cardiff Business School, Cardiff University , Colum Drive, Cardiff, CF10 3EU, Wales, UK E-mail: