1. Damla Şentürk is Assistant Professor, Department of Statistics, Pennsylvania State University, University Park, PA 16801 . Hans-Georg Müller is Professor, Department of Statistics, University of California, Davis, One Shields Avenue, Davis, CA 95616 . This research was supported in part by a grant from the National Science Foundation (DMS-08-06199). We are grateful to two referees and an Associate Editor for their constructive comments and careful reading.