Demystifying propensity scores.

1 Health Protection and Influenza Research Group, Division of Epidemiology and Public Health, University of Nottingham, Nottingham, UK 2 Department of Anaesthesia, National Hospital for Neurology and Neurosurgery, & Surgical Outcomes Research Centre, University College London Hospital, London, UK 3 Wellcome Department of Imaging Neuroscience, University College London, London, UK 4 Department of Anaesthesia, National Hospital for Neurology and Neurosurgery, London WC1N 3BG, UK