[1] Univ Penn, Dept Med, Div Gastroenterol, Philadelphia, PA 19104 USA
[2] Univ Penn, Ctr Clin Epidemiol & Biostat, Perelman Sch Med, Philadelphia, PA 19104 USA
[3] Univ Penn, Leonard Davis Inst, Philadelphia, PA 19104 USA
[4] Univ Texas Hlth Sci Ctr Houston, Dept Internal Med, Div Gastroenterol Hepatol & Nutr, Houston, TX 77030 USA