School of Medicine, Marsico Lung Institute, UNC Cystic Fibrosis Research Center, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America. Department of Pulmonology, Division of Pulmonary and Critical Care Medicine, School of Medicine, The University of Kansas, Kansas City, Kansas, United States of America. Department of Epidemiology and Biostatistics, School of Medicine, Case Western Reserve University, Cleveland, Ohio, United States of America. Department of Mathematics and Statistics, University of Wyoming, Laramie, Wyoming, United States of America. Physiology and Experimental Medicine, Research Institute, The Hospital for Sick Children, Toronto, Ontario, Canada. Division of Gastroenterology, Hepatology and Nutrition, The Hospital for Sick Children, University of Toronto, Toronto, Ontario, Canada. Department of Paediatrics, University of Toronto, Toronto, Ontario, Canada.