[1] Univ Penn, Perelman Sch Med, Ctr Clin Epidemiol & Biostat, Dept Biostat & Epidemiol, Philadelphia, PA 19104 USA
[2] Univ Penn, Leonard Davis Inst Hlth Econ, Philadelphia, PA 19104 USA
[3] Intermt Med Ctr, Dept Med, Div Hepatol, St Murray, UT USA
[4] Ctr Clin Epidemiol & Biostat, 730 Blockley Hall,423 Guardian Dr, Philadelphia, PA 19104 USA