1 Department of Epidemiology and International Public Health, School of Public Health, Bielefeld University, P. O. Box 100131, 33501, Bielefeld, Germany. oliver.razum@uni-bielefeld.de.
2 MRC/CSO Social and Public Health Sciences Unit, University of Glasgow, 200 Renfield Street, Glasgow, G2 3QB, UK.
3 Department of Public Health Nursing and Health Services Research, School of Public Health, Bielefeld University, P. O. Box 100131, 33501, Bielefeld, Germany.