1. Department of Social Policy and Intervention; University of Oxford; Oxford OX1 2ER UK
2. London School of Hygiene and Tropical Medicine; London WC1E 7HT UK
3. Institute for Medical Informatics, Biometry and Epidemiology; Ludwig-Maximilians-University; Munich 81377 Germany
4. RAND Corporation; Santa Monica CA 90407-2138 USA
5. Department of Social Policy, Sociology and Criminology; University of Birmingham; Edgbaston Birmingham B15 2TT UK