[1] Univ Michigan, Dept Family Med, Ann Arbor, MI 48109 USA
[2] Univ Penn, Hosp Univ Penn, Perelman Sch Med,Digest & Liver Ctr, Div Gastroenterol,GI Physiol Lab,Acid Pept Disord, Philadelphia, PA 19104 USA
[3] Univ Penn, Perelman Sch Med, Ctr Clin Epidemiol & Biostat, Philadelphia, PA 19104 USA