Footnotes

• Contributors All authors contributed to the concept and design of the study. KJ collated the data which were analysed by all authors. KJ and SK drafted the manuscript, which was revised and edited by all authors.

• Competing interests None.

• Ethics approval The protocol was reviewed by Dr Phillip Sedgwick, reader in Statistics and course organiser at St George's Hospital Medical School, University of London.

• Provenance and peer review Not commissioned; externally peer reviewed.