MRC Biostatistics Unit, Institute of Public Health, University Forvie Site, Cambridge CB2 0SR, UK. john.reid@mrc-bsu.cam.ac.uk|MRC Biostatistics Unit, Institute of Public Health, University Forvie Site, Cambridge CB2 0SR, UK. john.reid@mrc-bsu.cam.ac.uk|MRC Biostatistics Unit, Institute of Public Health, University Forvie Site, Cambridge CB2 0SR, UK. john.reid@mrc-bsu.cam.ac.uk