Richard D Neal1, Fei Sun2, Jon D Emery3, Matthew E Callister2. 1. Academic Unit of Primary Care, Leeds Institute of Health Sciences, University of Leeds, Leeds LS2 9NL, UK R.D.Neal@leeds.ac.uk. 2. Leeds Teaching Hospitals Trust, St James's Hospital, Leeds LS9 7TF, UK. 3. Centre for Cancer Research and Department of General Practice, Faculty of Medicine, Dentistry and Health Sciences, University of Melbourne, Victorian Comprehensive Cancer Centre, Victoria 3000, Australia.