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LETTERS TO THE EDITOR
On the need for the rare disease assumption in some case-control studies
  1. P Cummings,
  2. T D Koepsell
  1. Department of Epidemiology, School of Public Health and Community Medicine, University of Washington and the Harborview Injury Prevention and Research Center, 325 Ninth Ave, Box 359960, Seattle, Washington 98104–2499, USA peterc@u.washington.edu

    https://doi.org/10.1136/ip.7.3.254-a

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      Editor,—Several case-control studies have estimated the association between wearing of a bicycle helmet and head injury due to a bicycle crash.1 Hagel and Boivin recently argued that these studies do not need the rare disease assumption in order for us to accept the odds ratio as an estimate of the incidence rate ratio.2 In support of their argument, Hagel and Boivin described a hypothetical case-control study in which the study population is thought of as people riding bicycles.2 The cases were those who had a head injury after a crash. The controls were randomly sampled from all bicycle riders, regardless of whether or not they had just crashed. Such a study would seek to answer …

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