1. a Robertson Centre for Biostatistics , University of Glasgow, University Avenue , Glasgow, UK
2. b Division of Cardiovascular and Medical Sciences , University of Glasgow, Gardiner Institute, Western Infirmary , Glasgow, UK
3. c MRC Biostatistics Unit , Institute of Public Health , Cambridge, UK
4. d Division of Health and Social Care Research, Department of Public Health Sciences , School of Medicine, King's College , London, UK