1. Nuffield Department of Primary Care Health Sciences, University of Oxford, Radcliffe Primary Care Building, Radcliffe Observatory Quarter, Woodstock Rd, Oxford OX2 6GG, England.
2. Nuffield Department of Population Health, University of Oxford, Oxford, England.
3. Essex County Council, Essex, England.
4. Bodleian Health Care Libraries, University of Oxford, Oxford, England.
5. Primary Care and Population Sciences Unit, University of Southampton, Southampton, England.
6. Public Health Wales, Cardiff, Wales.