MRC/CSO Social and Public Health Sciences Unit, University of Glasgow, Glasgow
Funding: The authors are funded by the Medical Research Council, UK (MC_UU_12017/13) and the Chief Scientist Office of the Scottish Government Health Directorates (SPHSU 13).
Contributions: FP had the idea, which was developed with EW. Both authors contributed to writing the paper.
Conflicts of interest: No conflicts of interest
Data availability: The data are in the paper
MRC/CSO Social and Public Health Sciences Unit, University of Glasgow, 200 Renfield Street, Glasgow, G2 3QB, Tel: +44 0141 353 7500, firstname.lastname@example.org