1. University of North Carolina Department of Biostatistics, , 135 Dauer Drive, Chapel Hill, North Carolina 27599, U.S.A
2. North Carolina State University Department of Statistics, , 2311 Stinson Drive, Raleigh, North Carolina 27607, U.S.A
3. Department of Biostatistics, University of North Carolina Collaborative Studies Coordinating Center, , 123 W. Franklin Street, Suite 450, Chapel Hill, North Carolina 27516, U.S.A