Office of the Chief Economist, U.S. Securities and Exchange Commission and Department of Economics, University of Chicago. I have received helpful comments from many people. These include David Malmquist, Gregg Jarrell, Mike Ryngaert, Annette Poulson, Jeff Netter, Brandon Becker, Rick Ketchum, Jeff Davis, Lois Lightfoot, Charles Cox, and Joe Grundfest from the S.E.C.; Hans Stoll, Sy Smidt, and Cathy Bonser-Neal; and especially Charlie Kahn, Merton Miller, and Arnold Zellner. The SEC disclaims responsibility for any private publication by its employees. The views herein are those of the author and do not necessarily reflect the views of the SEC.