Footnotes

• Contributors HT is the guarantor. HT, DS, AM and SE conceived the idea. HT and MD determined the best comparator. DS and SE undertook the data collection and investigation. HT undertook the analysis. All authors contributed to the draft and revisions.

• 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.