Division of Gastroenterology and Hepatology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA. Division of Gastroenterology, Hepatology, and Nutrition, Department of Medicine, Vanderbilt University Medical Center, Nashville, Tennessee, USA. Department of Statistical Science, Duke University, Durham, North Carolina, USA. Department of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA. Health Sciences Library, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA. Division of Gastroenterology and Hepatology, University of Michigan, Ann Arbor, Michigan, USA. Gastroenterology Section, VA Ann Arbor Healthcare System, Ann Arbor, Michigan, USA.