Footnotes

• Contributors MM: conceived and designed the analysis, collected the data and wrote the paper. SS, FP and CC: reviewed the paper. All authors discussed the results and contributed to the final manuscript.

• 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 by William Whitely, Edinburgh, UK.