[1] So Calif Permanente Med Grp, Dept Res & Evaluat, Pasadena, CA 91101 USA
[2] Mayo Clin Rochester, Div Epidemiol, Rochester, MN USA
[3] Merck Res Labs, Dept Epidemiol, West Point, PA USA
[4] Mayo Clin Rochester, Div Biostat, Rochester, MN USA
[5] Mayo Clin Rochester, Dept Med, Rochester, MN USA