1. Living Standards Measurement Study, Development Data Group, World Bank, Rome 00184, Italy
2. Development Economics Group, Wageningen University and Research, Wageningen 6706KN, The Netherlands
3. Department of Civil and Environmental Engineering, Rice University, Houston, TX 77005
4. Department of Economics, Northwestern University, Evanston, IL 60208
5. Department of Economics, University of Oxford, Oxford OX1 3UQ, United Kingdom
6. Department of Economics, Tufts University, Medford, MA 02155