Footnotes

• Contributors All authors equally conceived the idea of the study and developed the statistical analysis plan and conducted statistical analyses. YI drafted the original manuscript. SH supervised the conduct of this study. All authors reviewed the manuscript draft and revised it critically on intellectual content. All authors approved the final version of the manuscript to be published.

• 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.