1. Department of Pediatrics, Children’s Hospital of Philadelphia, University of Pennsylvania, Philadelphia, Pennsylvania, USA
2. Biostatistics Branch and
3. Epidemiology Branch, National Institute of Environmental Health Sciences, National Institutes of Health, Department of Health and Human Services, Research Triangle Park, North Carolina, USA
4. Department of Neonatology, Children’s Hospital Medical Center, University of Cincinnati, Cincinnati, Ohio, USA
5. Survey & Epidemiology Services Division, Social & Scientific Systems, Inc. (formerly Coda, Inc.), Durham, North Carolina, USA
6. Westat Inc., Durham, North Carolina, USA