1. Department of Physical Medicine & Rehabilitation, Vienna Medical University, Vienna, Austria
2. Vienna Medical University, Vienna, Austria
3. Physical of Medicine & Rehabilitation, Salzburg Medical University, Salzburg, Austria
4. Department of Neurosurgery, Medical University of Vienna, Vienna, Austria
5. Section of Medical Statistics, Medical University of Vienna, Vienna, Austria
6. German Institute of Health Research, Bad Elster and Dresden, Germany