[1] Univ Calif San Francisco, Dept Epidemiol & Biostat, San Francisco, CA 94143 USA
[2] Kaiser Permanente Northern Calif, Div Res, Oakland, CA USA
[3] Massachusetts Gen Hosp, Clin Epidemiol Unit, Boston, MA 02114 USA
[4] Boston Univ, Sch Med, Gen Internal Med & Clin Epidemiol Unit, Boston, MA 02118 USA