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Bias in estimates of seat belt effectiveness
  1. T D Koepsell,
  2. F P Rivara,
  3. D C Grossman,
  4. C Mock
  1. Department of Epidemiology, Box 357236, University of Washington, Seattle, WA 98195, USA
  1. Correspondence to:
 Dr Koepsell;
 koepsell{at}u.washington.edu

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In his recent commentary entitled “Bias in estimates of seat belt effectiveness”,1 Robertson criticizes our study of seat and shoulder belts in relation to crash injury risk.2 He writes: “In one of the recent studies claiming high belt effectiveness, missing data on velocity changes in crashes were imputed partly from injury severity scores, again a cause imputed from an effect and then used as a control in the study, a true scientific ‘no-no’.”

Robertson’s criticism is incorrect. When multiple imputation is used to deal with missing data on a covariate, the imputation model needs to preserve relationships between that covariate and other key variables that will be used in the main analysis.3 These other key variables include both exposure and outcome. In contrast, Robertson argues that measures of crash outcome should not be used to impute values on a covariate which will later enter the main analysis as a predictor of crash outcome.

In our study, velocity change during the crash (delta-V) was a clear confounder: when known, larger delta-V was associated …

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