Health Services Research and Policy, London School of Hygiene & Tropical Medicine, London, UK Jack.Williams@lshtm.ac.uk. Clinical Trials Unit, London School of Hygiene & Tropical Medicine, London, UK. Centre for Urgent and Emergency Care Research, School of Health and Related Research, The University of Sheffield, Sheffield, UK. Emergency Department, Salford Royal Hospital NHS Foundation Trust, Salford, Salford, UK. Holy Family Hospital, Rawalpindi Medical University, Rawalpindi, Pakistan. Health Services Research and Policy, London School of Hygiene & Tropical Medicine, London, UK.