1. Department of Epidemiology, University of North Carolina Chapel Hill, Chapel Hill, North Carolina.
2. Department of Community and Preventive Medicine, Mount Sinai School of Medicine, New York, New York.
3. Present address: Division of Cancer Epidemiology and Genetics, National Cancer Institute, National Institutes of Health, DHHS, Rockville, Maryland.
4. Present address: Epidemiology Branch, National Institute of Child Health and Human Development, National Institutes of Health, DHHS, Rockville, Maryland.