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Contributors Both authors contributed to the drafting and writing of the manuscript.
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 CH and KRM are supported by the NIHR School for Primary Care Research Evidence Synthesis Working group (NIHR SPCR ESWG project 390). CH is also supported by the NIHR Oxford Biomedical Research Centre, is an NIHR Senior Investigator and Editor-in-Chief of BMJ Evidence-Based Medicine. KRM is Associate Editor of BMJ EBM.
Provenance and peer review Commissioned; internally peer reviewed.
Patient consent for publication Not required.
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