1. Department of Epidemiology & Public Health Medicine, University of Bristol, Canynge Hall, Whiteladies Road, Bristol, BS8 2PR
2. Avon Micro Consultancy, Bristol
3. Department of Medicine, Bristol Royal Infirmary
4. Department of Epidemiology and Public Health Medicine, University of Bristol, Canynge Hall, Whiteladies Road, Bristol, BS8 2PR