Footnotes

• Contributors NRR and JFL conceived and designed the study with input from PHRG. JFL collected the data, performed the statistics and funded the study. All authors interpreted the data and contributed to the writing of the paper. All authors revised and approved the final version.

• Funding All authors have completed the ICMJE uniform disclosure form at http://www.icmje.org/coi_disclosure.pdf and declare that this project was supported by grants from the Swedish Society of Medicine and the Swedish Research Council.

• Competing interests None declared.

• Ethics approval This project (2008/1182-31/4) was approved by the Research Ethics Committee of the Karolinska Institute, Sweden on 3 September 2008.

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

• Data sharing statement Other researchers can apply for our data through the Swedish National Board of Health and Welfare.

• Transparency The lead author affirms that this manuscript is an honest, accurate and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.