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Contributors Dr F Swann and Dr D Henderson contributed equally to the design, conduct and reporting of this work. Dr I Carey contributed to the design and reporting. Dr D Wakefield contributed to the design, conduct and reporting.
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; internally peer reviewed.