1. Department of General Internal Medicine and Psychosomatics, Medical Hospital, University of Heidelberg, Im Neuenheimer Feld 410, Heidelberg D-69120, Germany
2. Department for Methods of Community Medicine, Institute for Community Medicine, University Greifswald, Medicine Ernst-Moritz-Arndt-University Greifswald, Walther-Rathenau-Str. 48, Greifswald D-17487, Germany
3. Department of Mathematics and Statistics, Boston University, 111 Cummington Street, Boston, MA 02215, USA
4. Department of Ophthalmology, University Medical Center Mainz, Langenbeckstrasse 1, Mainz D-55131, Germany