1. Kenneth C. Griffin Department of Economics, University of Chicago, 1126 E 59th Street, Chicago, IL 60637, and NBER (email: )
2. Department of Economics, Stanford University, 579 Serra Mall, Stanford, CA 94305, and NBER (email: )
3. Department of Economics and Graduate School of Business, Stanford University, 579 Serra Mall, Stanford, CA 94305, and NBER (email: )
4. Microsoft, 1 Microsoft Way, Redmond, WA 98052 (email: )