1 Nuffield Department of Primary Care Health Sciences, University of Oxford, Oxford OX2 6GG, UK. Electronic address: brian.nicholson@phc.ox.ac.uk.
2 Test Evaluation Group, University of Leeds, Leeds, UK.
3 College of Medicine and Health, University of Exeter, Exeter, UK.
4 Nuffield Department of Primary Care Health Sciences, University of Oxford, Oxford OX2 6GG, UK; Department of Public Health and Primary Care, University of Leuven, Leuven, Belgium.
5 Medical School, University of Exeter, Exeter, UK.