1. Department of Economics, Massachusetts Institute of Technology, 50 Memorial Drive, E52-371, Cambridge, MA 02142, and National Bureau of Economic Research (NBER) (e-mail: )
2. Centre for Economic Performance and Department of Economics, London School of Economics, Houghton Street, London WC2A 2AE, United Kingdom (e-mail: )
3. Federal Reserve Board of Governors, 20th & C Street, NW, Washington DC 20551 (e-mail: )