The exposure term in the case-crossover design consists in the difference between the ambient concentration on the event day and the concentration(s) on some control day(s). So far, all air pollution case-crossover studies presented the distribution of the daily ambient pollutant concentrations but do not publish the distributional properties of the relevant exposure term—that is, the concentration difference. This article shows that this difference can be very small for a large fraction of event days, therefore, seriously limiting the statistical power to refute the null hypothesis. Publishing the distribution of the relevant differences will improve the interpretation and discussion of findings from case-crossover studies, particularly in cases with statistically non-significant associations.
- air pollution
- study design
- statistical power
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1. Details of the power calculation for the case-crossover-model.
2. Details of the power calculation for the Poisson-regression model.
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Funding: Professor Kuenzli is supported by the Southern California Environmental Health Sciences Center (National Institute of Environmental Health Sciences grant P30 ES07048) and the Hastings Foundation.
Conflicts of interest: none declared.
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