[1] Michael E DeBakey VA Med Ctr, Houston VA HSR&D Ctr Excellence, Houston, TX USA
[2] Baylor Coll Med, Dept Med, Houston, TX 77030 USA
[3] Childrens Hosp Philadelphia, Dept Gastroenterol, Philadelphia, PA 19104 USA
[4] Univ Penn, Ctr Clin Epidemiol & Biostat, Philadelphia, PA 19104 USA
[5] Univ Penn, Dept Med, Philadelphia, PA 19104 USA
[6] Univ Penn, Dept Biostat & Epidemiol, Philadelphia, PA 19104 USA