1 School of Health, Wellington Faculty of Health, Victoria University of Wellington, Wellington 6140, New Zealand; Usher Institute, The University of Edinburgh, Edinburgh, UK. Electronic address: colin.simpson@vuw.ac.nz.
2 Department of Mathematics and Statistics, University of Strathclyde, Strathclyde, UK; Public Health Scotland, Glasgow, UK.
3 Public Health Scotland, Glasgow, UK.
4 Centre for Academic Primary Care, University of Aberdeen, Aberdeen, UK.
5 Usher Institute, The University of Edinburgh, Edinburgh, UK.