1. National Bureau of Economic Research, Cambridge, MA 02138, USA.
2. Department of Economics, Stanford University, Stanford, CA 94305, USA.
3. Department of Economics, Massachusetts Institute of Technology, Cambridge, MA 02142, USA.
4. Department of Economics, Harvard University, Cambridge, MA 02138, USA.
5. Department of Emergency Medicine and Health Care Policy, Harvard Medical School, Boston, MA 02115, USA.