1. University Medical Center Göttingen Department of General Practice, , Göttingen D-37073 , Germany
2. University Medical Center Hamburg-Eppendorf Department of General Practice and Primary Care, , Hamburg D-20246 , Germany
3. University of Cologne and University Hospital Cologne Institute of Nursing Science, , Cologne D-50935 , Germany
4. Nursing Research Unit, University of Lübeck Institute for Social Medicine and Epidemiology, , Lübeck D-23538 , Germany
5. University Medical Center Göttingen Department of Medical Statistics, , Göttingen D-37073 , Germany