1. Department of Biostatistics; Vanderbilt University; Nashville TN 37232 U.S.A.
2. Department of Biomedical Informatics; Vanderbilt University; Nashville TN 37232 U.S.A.
3. The EMMES Corporation; Rockville MD 20850 U.S.A.
4. Department of Biostatistics; University of North Carolina at Chapel Hill; Chapel Hill NC 27599 U.S.A.
5. Division of Biostatistics, School of Public Health; University of Minnesota; Minneapolis MN 55455 U.S.A.
6. Department of Epidemiology; University of North Carolina at Chapel Hill; Chapel Hill NC 27599 U.S.A.