1. Centre for Statistical Methodology, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK
2. Centre for Statistical Methodology, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK and MRC Clinical Trial Trials Unit, Kingsway, London WC2B 6NH, UK
3. School of Social and Community Medicine, University of Bristol, Canynge Hall, 39 Whatley Road, Bristol BS8 2PS, UK
4. Department of Applied Mathematics, Computer Science and Statistics, Ghent University, Krijgslaan, 281 S9, B-9000 Ghent, Belgium