Author Contributions: Dr Schroter and Mr Hutchings 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: Schroter, Black.
Acquisition of data: Schroter, Tite.
Analysis and interpretation of data: Schroter, Hutchings.
Drafting of the manuscript: Schroter, Hutchings, Black.
Critical revision of the manuscript for important intellectual content: Schroter, Tite, Hutchings.
Statistical analysis: Hutchings.
Administrative, technical, or material support: Schroter, Tite.
Study supervision: Schroter, Black.
Financial Disclosures: None reported.
Funding/Support: This study was funded by the BMJ Publishing Group's research budget.
Previous Presentation: Presented in part at the Fifth International Congress on Peer Review and Biomedical Publication; September 16-18, 2005; Chicago, Ill.
Acknowledgment: We thank Michael Healy, retired statistical advisor emeritus, Archives of Disease in Childhood, and Ben Armstrong, reader in epidemiology statistics, London School of Hygiene & Tropical Medicine, for their unpaid and independent statistical advice.