Indiana University School of Medicine, Indianapolis, Indiana, USA. George Washington University Biostatistics Center, Rockville, Maryland, USA. University of Pittsburgh Medical Center, Children's Hospital of Pittsburgh, Pittsburgh, Pennsylvania, USA. Yale University School of Medicine, New Haven, Connecticut, USA. University of Colorado Anschutz Medical Campus, Children's Hospital Colorado, Aurora, Colorado, USA. University of Southern California, Keck School of Medicine, Los Angeles, California, USA. University of Chicago, Medicine, Chicago, Illinois, USA. VA Puget Sound Health Care System and University of Washington, Seattle, Washington, USA.