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Correction notice This paper has been updated since it was published online. The second author's surname was spelt incorrectly.
Contributors The concept of the paper was initiated by DJ and CVEP. DJ and IA completed the initial literature search and data extraction. DJ wrote the original manuscript which was then developed and edited by CVEP and IA.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; externally peer reviewed.
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