1 Nuffield Department of Primary Care Health Sciences, University of Oxford, Radcliffe Primary Care Building, Woodstock Road, Oxford, OX2 6GG, UK. jan.verbakel@phc.ox.ac.uk.
2 Department of Public Health and Primary Care, KU Leuven, Kapucijnenvoer 33 J, 3000, Leuven, Belgium. jan.verbakel@phc.ox.ac.uk.
3 Department of Family Medicine and Primary Health Care, Ghent University, De Pintelaan 185, 9000, Gent, Belgium.
4 Department of Public Health and Primary Care, KU Leuven, Kapucijnenvoer 33 J, 3000, Leuven, Belgium.
5 Leeds Institute of Health Sciences, University of Leeds, 101 Clarendon Road, LS29LJ, Leeds, UK.
6 Nuffield Department of Primary Care Health Sciences, University of Oxford, Radcliffe Primary Care Building, Woodstock Road, Oxford, OX2 6GG, UK.
7 Research Institute Caphri, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands.