Affiliation: | Michael R. Elliott is with the Department of Biostatistics and the Survey Methodology Program at the University of Michigan, Ann Arbor. Michael J. Kallan is with the Center for Epidemiology and Biostatistics at the University of Pennsylvania, Philadelphia. Dennis R. Durbin is with the Center for Epidemiology and Biostatistics and the Division of Pediatric Emergency Medicine at the University of Pennsylvania, Philadelphia. |