1. Institute of Health and WellBeing, University of Glasgow, 1 Horselethill Road, Glasgow, G12 9LX, Scotland.
2. Faculty of Health Sciences, University of Southampton, Southampton, England.
3. Institute of Health and Society, Newcastle University, Newcastle-upon-Tyne, England.
4. Population Health Sciences, Medical Research Institute, University of Dundee, Dundee, Scotland.
5. Research Department of Primary Care and Population Health, University College London, London, England.