Footnotes

• X @nil, @katehodg18, @drbretty

• Contributors The research question for this study was conceived by BJM and developed by TAM, KAH and BJM. TAM undertook the data extraction and data analysis. TAM and KAH drafted the first draft of the manuscript. All authors were involved in interpretation, review and revision of the draft manuscript and approval of the final version. The authors had full access to the data in the study and take full responsibility for the integrity of the data and accuracy of the data analysis. The authors affirm that the manuscript is an honest, accurate and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned have been explained.

• 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.

• Provenance and peer review Not commissioned; externally peer-reviewed.