1. Michael G. Hudgens is Research Associate Professor, Department of Biostatistics, University of North Carolina, Chapel Hill, NC 27599 . M. Elizabeth Halloran is Professor, Program in Biostatistics and Biomathematics, Fred Hutchinson Cancer Research Center, and Department of Biostatistics, University of Washington, Seattle, WA 98185 . MGH and MEH were supported in part by NIH grant 2 R01 AI32042. The authors thank Peter Gilbert and Thomas Richardson for helpful comments.