1. Lund University, Faculty of Medicine, Department of Clinical Sciences-Lund, Orthopaedics, Clinical Epidemiology Unit, Lund, Sweden
2. The Parker Institute, Bispebjerg and Frederiksberg Hospitals, University of Copenhagen, Copenhagen, Denmark
3. Department of Rheumatology, Lund University Hospital, Lund, Sweden
4. Clinical Epidemiology Research and Training Unit, Boston University School of Medicine, Boston, MA, USA