1. Dylan S. Small is Assistant Professor, Department of Statistics, The Wharton School, University of Pennsylvania, Philadelphia, PA 19104 . Thomas R. Ten Have is Professor, Department of Biostatistics and Epidemiology, School of Medicine, University of Pennsylvania, Philadelphia, PA 19104 . Paul R. Rosenbaum is Professor, Department of Statistics, The Wharton School, University of Pennsylvania, Philadelphia, PA 19104 .