Footnotes

• Contributors All authors have made substantial contributions to the conception or design of the work and drafting of the work/revising it critically for important intellectual content. Once the final version was sent around, they all approved the final version which will be published and in doing so agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

• Competing interests None declared

• Provenance and peer review Not commissioned; externally peer reviewed.

• Correction notice This paper has been amended since it was published Online First. Owing to a scripting error, some of the publisher names in the references were replaced with ’BMJ Publishing Group'. This only affected the full text version, not the PDF. We have since corrected these errors and the correct publishers have been inserted into the references.