Author Contributions: Ms Horsfall and Dr Petersen had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.
Study concept and design: Horsfall, Rait, Walters, Swallow, Pereira, Nazareth, Petersen.
Acquisition of data: Horsfall.
Analysis and interpretation of data: Horsfall, Walters, Swallow, Pereira, Nazareth, Petersen.
Drafting of the manuscript: Horsfall.
Critical revision of the manuscript for important intellectual content: Rait, Walters, Swallow, Pereira, Nazareth, Petersen.
Statistical analysis: Horsfall, Petersen.
Obtained funding: Nazareth.
Administrative, technical, or material support: Rait, Nazareth.
Study supervision: Rait, Walters, Swallow, Pereira, Petersen.
Conflict of Interest Disclosures: All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and none were reported.
Funding/Support: Ms Horsfall completed this work as part of her post at the University College London Department of Primary Care and Population Health with additional funding from the National Institute for Health Research School for Primary Care Research. Dr Petersen received funding through grants G0601726 and G0900701 from the UK Medical Research Council.
Role of the Sponsor: The external funders had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; or preparation, review, or approval of the manuscript.