Footnotes

• Contributors OM had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Concept and design: all authors. Acquisition, analysis or interpretation of data: all authors. Drafting of the manuscript: OM. Critical revision of the manuscript for important intellectual content: K-HY, RW-M, IK and ND. Statistical analysis: OM, IK and K-HY.Study supervision: IK and ND.

• 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.