3 Institute of Population Health Sciences, Queen Mary University of London, London, UK.
4 Department of General Practice, RCSI University of Medicine and Health Sciences, Dublin, Ireland.
5 Academic Unit of Primary Medical Care, University of Sheffield, Northern General Hospital, Sheffield, UK.
6 Institute of Health and Wellbeing, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow G12 9LX, UK. Electronic address: graham.watt@glasgow.ac.uk.