1. Children's Hospital of Philadelphia and University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania.
2. University of South Florida College of Medicine, St. Petersburg, Florida.
3. National Institute of Child Health and Human Development, Bethesda, Maryland.
4. Department of Epidemiology, School of Public Health, University of Alabama, Birmingham, Alabama.
5. Westat, Rockville, MD, Maryland.
6. Duke University School of Medicine, Durham, North Carolina.