1. Danish Headache Center, Department of Neurology, Rigshospitalet -Glostrup, Faculty of Health and Medical Sciences, University of Copenhagen, Glostrup, Denmark
2. Centre for Evidence-Based Medicine Odense (CEBMO) and Cochrane Denmark, Department of Clinical Research, University of Southern Denmark, Denmark
3. Open Patient data Exploratory Network (OPEN), Odense University Hospital, Odense, Denmark
4. Institute Medical Informatics, Biometry and Epidemiology, Medical Faculty of the University Duisburg-Essen, Essen, Germany.