Here are the references – it will be up here probably tomorrow.

http://www.ncbi.nlm.nih.gov/pubmed/22495689

http://www.ncbi.nlm.nih.gov/pubmed/9783861

http://www.bmj.com/content/314/7086/1037

http://onlinelibrary.wiley.com/doi/10.1002/hec.1242/abstract;jsessionid=3D0C17206FD3C96227DFD224D616F5FA.d02t03

http://www.oecd.org/dataoecd/40/24/45760738.pdf

http://onlinelibrary.wiley.com/doi/10.1002/hec.1242/pdf

http://www.york.ac.uk/res/herc/documents/wp/11_21.pdf

http://www.bmj.com/content/313/7072/1577?view=long&pmid=8990990

http://www.biomedcentral.com/content/pdf/1471-2466-10-41.pdf