1. Centre for Statistics in Medicine, Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences University of Oxford Windmill Road Oxford OX3 7LD U.K
2. Departments of Anesthesia and Clinical Epidemiology and Biostatistics Michael DeGroote School of Medicine, Faculty of Health Sciences, McMaster University and the Perioperative Research Group, Population Health Research Institute Hamilton Canada