1 Institute of Biomedical and Clinical Science, Royal Devon and Exeter Hospital, University of Exeter Medical School, Exeter, UK.
2 Health Statistics Group, Institute of Health Research, Royal Devon and Exeter Hospital, University of Exeter Medical School, Exeter, UK.
3 Institute of Biomedical and Clinical Science, Royal Devon and Exeter Hospital, University of Exeter Medical School, Exeter, UK. Electronic address: a.t.hattersley@exeter.ac.uk.