1. Qi Long is Rollins Assistant Professor, Department of Biostatistics, Emory University, Atlanta, GA 30329 . Roderick J. Little is Professor, Department of Biostatistics, University of Michigan, Ann Arbor, MI 48109 . Xihong Lin is Professor, Department of Biostatistics, Harvard University, Boston, MA 02115 . This research was supported by National Cancer Institute grant R01CA76404. The authors thank Noreen Clark for providing the WTP data and useful comments.