1. Muirhouse Medical Group, 1 Muirhouse Avenue, Edinburgh EH4 4PL, Scotland, UK
2. Division of Community Health Sciences, University of Edinburgh, Teviot Place, Edinburgh EH8 9AG, Scotland, UK
3. National Centre in HIV Epidemiology and Clinical Research, University of New South Wales, Sydney 2052, Australia
4. Department of Social Medicine, University of Bristol, Canynge Hall, 39 Whatley Road, Bristol BS8 2PS, England, UK
5. MRC Biostatistics Unit, Institute of Public Health, Robinson Way, Cambridge CB2 2SR, England, UK