1. Department of Health Policy, London School of Economics and Political Science, Houghton Street, Holborn, London WC2A 2AE, England.
2. School of Epidemiology and Public Health, University of Ottawa, Ottawa, Canada.
3. Department of Health Services Research and Policy, London School of Hygiene & Tropical Medicine, London, England.
4. Carlo F Dondena Centre for Research on Social Dynamics and Public Policy, Bocconi University, Milan, Italy.