1 University of Nottingham, Division of Epidemiology and Public Health, Clinical Sciences Building, City Hospital, Nottingham NG5 1PB, UK. Electronic address: j.britton@virgin.net.
2 University of Nottingham, Division of Epidemiology and Public Health, Clinical Sciences Building, City Hospital, Nottingham NG5 1PB, UK.
3 Action on Smoking and Health, London, UK.
4 Cancer Research UK Health Behaviour Unit, Department of Epidemiology and Public Health, University College London, London, UK.
5 Department of Health Sciences, University of York, York, UK.