Footnotes

• Contributors LSTM and ACW (study guarantor) performed the research, analysed the data and wrote the paper; ADA analysed the data and revised the manuscript critically. All authors had full access to all of the data (including statistical reports and tables) in the study and can take responsibility for the integrity and accuracy of the data.

• Competing interests All authors (LSTM, ADA and ACW) have completed the Unified Competing Interest form at http://www.icmje.org/coi_disclosure.pdf and declare no financial support/sponsors for the submitted work; LSTM, ADA and ACW declare no relationships that might have an interest in the submitted work in the previous 3 years; spouses, partners or children have no financial relationships that may be relevant to the submitted work; and LSTM, ADA and ACW have no financial interests that may be relevant to the submitted work.

• Provenance and peer review Not commissioned; internally peer reviewed.

• Transparency declaration The lead author declares 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 have been explained.