1. Department of Public Health, University of Aberdeen Medical School, Aberdeen, AB25 2ZD, U.K.
2. Department of Primary Care, University of Liverpool Medical School, Liverpool, L69 3GB, U.K.
3. Liverpool Primary Care Trust, Liverpool L10 1LD, U.K.
4. Halton & St. Helens Primary Care Trust, St. Helens, Merseyside WA10 2AP, U.K.