1 Division of Gastroenterology, Massachusetts General Hospital and Harvard Medical School, Clinical and Translational Epidemiology Unit, Massachusetts General Hospital, Boston, Massachusetts.
2 Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden; Department of Medicine, Columbia University College of Physicians and Surgeons, New York, New York; Department of Paediatrics, Örebro University Hospital, Örebro, Sweden.