1. Population, Policy and Practice Programme, UCL Great Ormond Street Institute of Child Health, 30 Guilford Street, London WC1N 1EH, UK
2. Research Department of Primary Care and Population Health, University College London, Royal Free Campus, Rowland Hill Street, London NW3 2PF, UK
3. Statistics Group, Department of Mathematics and Statistics, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
4. PRIMENT Clinical Trials Unit, Research Department of Primary Care and Population Health, University College London, Royal Free Campus, Rowland Hill Street, London NW3 2PF, UK
5. Department of Clinical Epidemiology, Aarhus University, Olof Palmes Allé 43-45, DK 8200 Aarhus N, Denmark