1. Institute of Applied Health Research, University of Birmingham , Birmingham , UK
2. School of Public Health, Imperial College London , London , UK
3. Nuffield Department of Primary Care Health Sciences, University of Oxford , Oxford , UK
4. Department of Medical Statistics, London School of Hygiene and Tropical Medicine , London , UK
5. MRC Biostatistics Unit, University of Cambridge , Cambridge , UK
6. University of Edinburgh College of Medicine and Veterinary Medicine , Edinburgh , UK