1. Section of Paediatrics, Imperial College London, London, England2Respiratory Epidemiology, Imperial College London, London, England
2. Respiratory Epidemiology, Imperial College London, London, England
3. Section of Paediatrics, Imperial College London, London, England
4. Institute of Medical Sciences, University of Aberdeen, Aberdeen, Scotland
5. University Division of Population Medicine, Cardiff University, Cardiff, Wales
6. Centre for Statistics in Medicine, University of Oxford, Oxford, England
7. Division of Epidemiology and Public Health, University of Nottingham, Nottingham, England