1. Department of Pediatrics, University of Iowa, Iowa City, Iowa;
2. Statistics and Epidemiology Unit, RTI International, Research Triangle Park, North Carolina;
3. Department of Pediatrics, Emory University, Atlanta, Georgia;
4. Department of Pediatrics, School of Medicine, University of Alabama at Birmingham, Birmingham, Alabama;
5. Department of Pediatrics, Stanford University, Palo Alto, California;
6. Department of Pediatrics, Brown University, Providence, Rhode Island;
7. Department of Pediatrics, Case Western Reserve University, Cleveland, Ohio;
8. Department of Pediatrics, Wayne State University, Detroit, Michigan;
9. Statistics and Epidemiology Unit, RTI International, Rockville, Maryland; and
10. Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland