1 Population Health Sciences, Bristol Medical School, University of Bristol, Bristol, UK D.J.Gunnell@bristol.ac.uk.
2 National Institute of Health Research Biomedical Research Centre at the University Hospitals Bristol NHS Foundation Trust and the University of Bristol, Bristol, UK.
3 Population Health Sciences, Bristol Medical School, University of Bristol, Bristol, UK.
4 National Institute for Health Research Applied Research Collaboration West (NIHR ARC West), University Hospitals Bristol NHS Foundation Trust, Bristol, UK.