1 Nuffield Department of Surgical Sciences, University of Oxford, UK; Department of Surgery, Geneva University Hospital, Switzerland. Electronic address: baptiste.vasey@lincoln.ox.ac.uk.
2 Centre for Statistics in Medicine, UK EQUATOR Centre, Nuffield Department of Orthopaedics, Rheumatology, and Musculoskeletal Sciences, University of Oxford, UK. Electronic address: http://www.twitter.com/GSCollins.