1Center for Injury Research and Prevention at The Children's Hospital of Philadelphia, Philadelphia, PA
2Division of General Pediatrics, Department of Pediatrics, University of Pennsylvania, Philadelphia, PA
3Division of Pediatric Emergency Medicine, Department of Pediatrics, University of Pennsylvania, Philadelphia, PA
4Center for Clinical Epidemiology and Biostatistics, University of Pennsylvania, Philadelphia, PA
5Department of Biostatistics, and Institute for Social Research, University of Michigan, Ann Arbor