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Contributors The study was conceptualised and designed by Dr MHE. All of the authors participated in data extraction, reviewed drafts of the manuscript and approved the final work.
Competing interests MHE serves as editor-in-chief of Essential Evidence, for which he is paid by Wiley-Blackwell. He is also one of the co-developers of the SORT framework for grading evidence.
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement We would be happy to collaborate with interested researchers and share our data with them as part of such a collaboration.