1. The EMMES Corporation; Rockville Maryland 20850 U.S.A.
2. Division of Biostatistics, School of Public Health; University of Minnesota; Minneapolis Minnesota 55455 U.S.A.
3. Department of Biostatistics; University of North Carolina at Chapel Hill; Chapel Hill North Carolina 27599 U.S.A.
4. Division of Infectious Diseases, Department of Medicine; University of North Carolina at Chapel Hill; Chapel Hill North Carolina 27599 U.S.A.
5. Department of Medicine, Epidemiology, Microbiology and Immunology; University of North Carolina at Chapel Hill; Chapel Hill North Carolina 27599 U.S.A.