This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/2.5/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Published by Oxford University Press in association with The London School of Hygiene and Tropical Medicine Health Policy and Planning 2012;27:326–338
ß The Author 2011; all rights reserved. Advance Access publication 7 June 2011
doi:10.1093/heapol/czr044