1. Division of Nephrology and Hypertension, Department of Medicine, University of North Carolina at Chapel Hill, North Carolina, USA
2. University of North Carolina Kidney Center, Chapel Hill, North Carolina, USA
3. Department of Epidemiology, University of North Carolina at Chapel Hill, North Carolina, USA
4. Statistics and Public Health Research Division, Constella Group, Inc., Durham, North Carolina, USA
5. Department of Biostatistics, University of North Carolina at Chapel Hill, North Carolina, USA
6. Departments of Epidemiology, Biostatistics and Medicine, The Johns Hopkins University, Baltimore, Maryland, USA