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Contributors CH wrote the first draft and KRM approved the final draft.
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 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.
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