1. Department of Management Science and Information Systems, Guanghua School of Management, Peking University, 100871 Beijing, China;
2. Robert H. Smith School of Business and Institute for Systems Research, University of Maryland, College Park, Maryland 20742;
3. Department of Econometrics and Operations Research, Vrije Universiteit Amsterdam, 1081 HV Amsterdam, Netherlands;
4. Department of Industrial Engineering and Operations Research, Columbia University, New York, New York 10027