1. Information Systems and Operations Management Department, Ecole des Hautes Etudes Commerciales Paris, 78350 Jouy-en-Josas, France;
2. Columbia Business School, Columbia University, New York, New York 10027;
3. Industrial Engineering and Operations Research Department, Columbia University, New York, New York 10027;
4. Kaiser Permanente, Division of Research, Oakland, California 94612