1. Unit of Periodontology, International Centre for Evidence-Based Oral Health, UCL Eastman Dental Institute, 256 Gray’s Inn Road, London WC1X 8LD, UK;
2. Chalmers Research Group, CHEO Research Institute; Department of Epidemiology and Community Medicine, Faculty of Medicine, University of Ottawa, ON, Canada;
3. Centre for Statistics in Medicine, University of Oxford, UK;
4. Family Health International, Research Triangle Park, North Carolina, USA;
5. Health Services Research, UCL Eastman Dental Institute, London, UK; and