1. University of Washington, 311 Savery Hall, UW Economics Box 353330, Seattle, WA 98195, and NBER (email: )
2. Department of Economics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 (email: )
3. Department of Economics, University of Chicago, 1126 East 59th Street, Chicago, IL 60637, and NBER (email: )
4. Amazon Inc., 345 Boren Avenue North, Seattle, WA 98109 (email: )