Willi Zhang, Per Tynelius, Gunnar Brandén, and Kyriaki Kosidou are with the Department of Global Public Health, Karolinska Institutet, and the Centre for Epidemiology and Community Medicine, Region Stockholm, Stockholm, Sweden. Maya B. Mathur is with the Quantitative Sciences Unit, Stanford University, Palo Alto, CA. Matteo Quartagno is with the MRC Clinical Trials Unit, University College London, London, England. Fredrik Liljeros is with the Department of Sociology, Stockholm University, and the Department