Department of Statistics, North Carolina State University, Raleigh, NC 27695-8203, USA.|Department of Healthcare Policy and Research, Weill Cornell Medical College, New York, NY 10065, USA.|Department of Biostatistics, University of Michigan, Ann Arbor, MI 48109-2029, USA.|Department of Biostatistics and Bioinformatics, Emory University, Atlanta, GA 30322, USA.