1. University of Oxford; Nuffield Department of Population Health; Oxford UK
2. University of Alberta; Department of Emergency Medicine; 790 University Terrace Building 8303 - 112 Street Edmonton Alberta Canada T6G 2T4
3. Norwegian Institute of Public Health; Division of Health Services; Marcus Thranes gate 6 Oslo Norway 0403
4. University of Oxford; Nuffield Department of Primary Care Health Sciences; Radcliffe Observatory Quarter Woodstock Road Oxford UK OX2 6GG