1. Division of Epidemiology, Statistics and Prevention Research, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Department of Health and Human Services, Rockville, Maryland, USA
2. Department of Epidemiology and Biostatistics, School of Rural Public Health, Texas A & M Health Science Center, College Station, Texas, USA
3. Department of Obstetrics and Gynecology, College of Medicine, The Ohio State University, Columbus, Ohio, USA
4. The EMMES Corporation, Rockville, Maryland, USA
5. Department of Occupational and Environmental Health, Rollins School of Public Health, Emory University, Atlanta, Georgia, USA