Footnotes

• Contributors WBK contributed to the idea, conception and analysis. EHT and RBL contributed to the data collection. AK contributed to the supervision and revision. All the authors reviewed the manuscript. The guarantor is WBK.

• 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.