D J Hunter1, R Rushmer2, A Best3. 1. Centre of Public Policy and Health, School of Medicine Pharmacy and Health, Wolfson Building, Durham University, Queens Campus, Thornaby, Stockton on Tees TS17 6BH, UK. Electronic address: d.j.hunter@durham.ac.uk. 2. School of Health and Social Care, Teesside University, Parkside West, Middlesbrough TS1 3BA, UK. Electronic address: r.rushmer@tees.ac.uk. 3. InSource Research Group, Centre for Clinical Epidemiology and Evaluation, Vancouver Coastal Health Research Institute, 6975 Marine Drive, West Vancouver, BC V7W 2T4, Canada. Electronic address: allan.best@in-source.ca.