1. a Glaxo Wellcome , Clinical Pharmacology Data Sciences, Greenford Road, Greenford, London, UK
2. b Institute of General Practice and Primary Care, School of Health and Related Research, University of Sheffield, Community Sciences Centre, Northern General Hospital , Sheffield, S5 7AU, UK
3. c ICRF Medical Statistics Laboratory , Centre for Statistics in Medicine, Institute of Health Sciences , Old Road, Headington, Oxford, OX3 7LF, UK