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