[1] Univ Pittsburgh, Dept Biostat, Pittsburgh, PA 15260 USA
[2] Vanderbilt Univ Sch Med, Dept Biostat, Nashville, TN USA
[3] Univ Penn, Perelman Sch Med, Div Gastroenterol & Hepatol, Philadelphia, PA USA
[4] Univ Penn, Perelman Sch Med, Dept Biostat Epidemiol & Informat, Philadelphia, PA 19104 USA