1 Pain Research and Nuffield Department of Anaesthetics, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DU, UK Centre for Pain Research, School for Health, The University of Bath, Bath BA2 7AY, UK The UK Cochrane Centre, NHS R&D Programme, Summertown Pavilion, Middle Way, Oxford OX2 7LG, UK Pain Clinic/Regional Centre of Excellence in Palliative Care, Haukeland University Hospital, Bergen, Norway Department of Occupational and Social Medicine, University of Göttingen, Waldweg 37, D-37073 Göttingen, Germany.