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Living in the grey area: a case for data sharing in observational epidemiology
  1. Brian D Johnston
  1. Correspondence to Dr Brian D Johnston, Department of Pediatrics, Harborview Medical Center, University of Washington, 325 Ninth Ave, Box 359774, Seattle, WA 98104, USA; ipeditor{at}bmjgroup.com

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In 2001, Injury Prevention journal published a paper by Macpherson and colleagues looking at the effect of a bicycle helmet ordinance on bicycle ridership in East York, Ontario, Canada.1 The questions addressed by the authors are important. While helmet laws may be enacted to promote helmet use and reduce bicycle-related injury, they could easily have unintended and undesirable consequences. Critics point out that helmet laws may send a message that cycling is inherently dangerous and could dissuade riders. In addition, there may be others who will choose not to ride if wearing a helmet is perceived as uncomfortable, cumbersome or costly. These potential effects on ridership must be acknowledged, measured and—if needed—mitigated. Active transportation through cycling is a promising strategy for building physical activity into daily life, directly addressing the public health burden of overweight and obesity, and reducing carbon emissions.2 Thus, any benefits from a law intended to prevent injury must be weighed against its effects on competing public health priorities and the interests of private citizens. Macpherson's paper measured ridership in one area before and after enactment of a helmet law and suggested that, in fact, the law had very little impact on the number of child cyclists. These data were also the focus of her 2003 PhD thesis.3

In 2003, Robinson published a research letter in Injury Prevention critical of Macpherson's findings.4 She argued that the study was sited in a jurisdiction where helmet laws were not enforced, that the total number of observations was small and that substantial year-to-year variability in ridership might be driven by variation in sites chosen for observation, date and time of day observed, and average weather conditions. Macpherson and colleagues, in a reply, attempted to address these concerns and called for more study. In a …

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