1. Office of Population Research, Princeton University, Princeton, NJ, 08540, USA
2. Department of Demography, University of California, Berkeley, CA, 94720, USA
3. Department of Health, Center for Population and Health Survey Research, Bureau of Health Promotion, Taichung City, 408, Taiwan, ROC
4. Center for Population and Health, Georgetown University, Washington, DC, 20057-1197, USA