1 Department of Biostatistics, The University of Liverpool, Liverpool, UK. Electronic address: sjn16@liverpool.ac.uk.
2 Centre for Clinical Practice, National Institute for Health and Care Excellence, Manchester, UK.
3 Centre for Respiratory Medicine and Allergy, Institute of Inflammation and Repair, University of Manchester and University Hospital of South Manchester, Manchester, UK.
4 Sydney Medical School, University of Sydney, Sydney, Australia.