1. Leeds Institute of Health Sciences, University of Leeds, Leeds, UK
2. Department of Epidemiology, Umm Al-Qura University, Makkah, Saudi Arabia
3. Department of Respiratory Medicine, Leeds Teaching Hospitals, St James's University Hospital, Leeds, UK
4. Center for Evaluation and Methods, Wolfson Institute of Population Health, Queen Mary University of London, London, UK
5. Department of Behavioural Science and Health, University College London, London, UK
6. College of Medicine and Health, University of Exeter, Exeter, UK