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Contributors CH wrote the first draft, and KRM contributed to the redrafting.
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.
Disclaimer The views expressed are those of the authors and not necessarily those of the National Health Service (NHS), the National Institute for Health Research (NIHR) or the Department of Health and Social Care.
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, Oxford University Hospitals NHS Foundation Trust, an NIHR senior investigator and editor in chief of BMJ Evidence-Based Medicine (BMJ EBM). KRM is an associate editor of BMJ EBM.
Provenance and peer review Commissioned; internally peer reviewed.
Patient consent for publication Not required.
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