1. Department of Management Science and Engineering, School of Engineering, Stanford University, Huang Engineering Center, 475 Via Ortega Avenue, Stanford, CA 94305 (email: )
2. Department of Economics, Columbia University, 1022 International Affairs Building, 420 West 118th Street, New York, NY 10027 (email: )
3. Department of Economics, Stanford University, 579 Serra Mall, Stanford, CA 94305 (email: )
4. Office of the Chief Technology Officer, Microsoft Corporation, One Microsoft Way, Redmond, WA 98052 (email: )
5. Microsoft Research, One Memorial Drive, Cambridge, MA 02142 and Yale University Department of Economics and Law School (email: )