1. Health Services Research, Faculty of Health, School of Medicine, Witten/Herdecke University, 58455 Witten, Germany
2. Department of Epidemiology and International Public Health, School of Public Health, Bielefeld University, 33615 Bielefeld, Germany
3. Department of Nursing and Health Services Research, School of Public Health, Bielefeld University, 33615 Bielefeld, Germany