1. Department of Biostatistics and Bioinformatics; Duke University School of Medicine; Durham 27710 NC U.S.A.
2. Duke Clinical Research Institute; Durham NC 27705 U.S.A.
3. National Institutes of Health, Office of Disease Prevention; Rockville MD 20892 U.S.A.
4. Department of Biostatistics; University of Washington; Seattle WA 98195 U.S.A.