1. School of Geography and the Environment , University of Oxford, OX1 3QY, UK
2. Institute of Health Informatics , University College London, NW1 2DA, UK
3. Department of Geography and Planning , University of Liverpool, L69 3GP, UK
4. Demography Unit , Sociology Department, Stockholm University, 106 91 Stockholm, UK
5. Department of Health Services Research and Policy , London School of Hygiene and Tropical Medicine, WC1H 9SH, UK