Footnotes

• Contributors SS and AD equally contributed in planning, conducting and reporting of the work in this paper. Conception and interpretation were done by FG and AF, who both supervised the work. SV and CK provided substantial contribution in writing and in final editing of the paper.

• Funding This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

• Competing interests None declared.

• Provenance and peer review Commissioned; externally peer reviewed.

• Correction notice This paper has been amended since it was published Online First. Owing to a scripting error, some of the publisher names in the references were replaced with ‘BMJ Publishing Group’. This only affected the full text version, not the PDF. We have since corrected these errors and the correct publishers have been inserted into the references.