1. University College London (UCL); Department of Epidemiology & Public Health; 1-19 Torrington Place London UK WC1E 6BT
2. Institute for Quality and Efficiency in Health care; Department of Health Information; Dillenburger Street 27, D-51105 Köln Germany D-51105
3. School of Dentistry, The University of Manchester; Department of Oral and Maxillofacial Surgery; Higher Cambridge Street Manchester UK M15 6FH