1. Department of Biostatistics; The University of North Carolina at Chapel Hill; Chapel Hill, North Carolina 27599 U.S.A.
2. Carolina Population Center; The University of North Carolina at Chapel Hill; Chapel Hill, North Carolina 27599 U.S.A.
3. Department of Statistics; Texas A&M University; College Station, Texas 77843 U.S.A.
4. Department of Epidemiology; The University of North Carolina at Chapel Hill; Chapel Hill, North Carolina 27599 U.S.A.
5. Departments of Statistical Science, Electrical and Computer Engineering, and Mathematics; Duke University; Durham, North Carolina 27708 U.S.A.