1. Center for Health Services Research in Primary Care; Durham VAMC; Durham NC U.S.A.
2. Department of Biostatistics; University of North Carolina; Chapel Hill NC 27599-7420 U.S.A.
3. Department of Biostatistics and Bioinformatics; Duke University School of Medicine; Durham NC U.S.A.
4. Division of General Internal Medicine; Department of Medicine; Duke University Medical Center; Durham NC U.S.A.