1. Division of Epidemiology, Statistics, and Prevention Research; Eunice Kennedy Shriver National Institute of Child Health and Human Development; National Institutes of Health; Rockville; MD; USA
2. Department of Biostatistics; University of Copenhagen; Copenhagen; Denmark
3. Department of Epidemiology and Biostatistics; University of South Carolina; Columbia; SC; USA
4. Population Dynamics Branch; Eunice Kennedy Shriver National Institute of Child Health and Human Development; National Institutes of Health; Rockville; MD; USA