1. Department of Epidemiology and Biostatistics, School of Public Health; Indiana University Bloomington; 1025 E. 7th street, Bloomington, Indiana, 47405 U.S.A.
2. Department of Epidemiology and Biostatistics, College of Public Health; University of Georgia; Athens, Georgia 30602 U.S.A.
3. Department of Biostatistics, College of Public Health & Health Professions and College of Medicine; University of Florida; Gainesville, Florida 32611 U.S.A.