1. Neil Pearce is with the London School of Hygiene and Tropical Medicine (LSHTM), London, UK. Jan P. Vandenbroucke is with the Leiden University Medical Center, Leiden, Netherlands, Aarhus University, Aarhus, Denmark, and the LSHTM. Tyler J. VanderWeele is with the Harvard T. H. Chan School of Public Health, Boston, MA. Sander Greenland is with the Departments of Epidemiology and Statistics, University of California, Los Angeles.