1 International Centre for Evidence in Disability, London School of Hygiene & Tropical Medicine, London WC1E 7HT, UK.
2 International Centre for Evidence in Disability, London School of Hygiene & Tropical Medicine, London WC1E 7HT, UK. Electronic address: tom.shakespeare@lshtm.ac.uk.
3 University College of Medical Sciences, University of Delhi, Delhi, India.
4 International Centre for Evidence in Disability, London School of Hygiene & Tropical Medicine, London WC1E 7HT, UK; Department of Health and Rehabilitation Sciences, Stellenbosch University, South Africa.