About the Authors

Robert Froud

r.froud@warwick.ac.uk

Affiliations Clinical Trials Unit, Warwick Medical School, University of Warwick, Gibbet Hill Road, Coventry, United Kingdom, Norge Helsehøyskole, Campus Kristiania, Prinsens Gate 7–9, Oslo, Norway

Gary Abel

Affiliation Cambridge Centre for Health Services Research, University of Cambridge, Robinson Way Cambridge, Cambridgeshire, United Kingdom

Competing Interests

GA declares that he has no competing interests. RF is also a director and shareholder of a company that provides electronic measurement services to health services researchers; notwithstanding this, he declares that he has no conflicts of interest. This does not alter the authors' adherence to PLOS ONE policies on sharing data and materials.

Author Contributions

Conceived and designed the experiments: RF GA. Performed the experiments: RF GA. Analyzed the data: RF GA. Wrote the first draft of the manuscript: RF. Commented on successive drafts: GA. Created the figures: GA. Wrote initial rocmic Stata module and help file: RF. Contributed to modifications of the updated code: GA.