The Conditional Case-crossover Design William Navidi Colorado School of Mines Crossover studies are experiments in which subjects pass through both treatment and placebo phases. Only cases are sampled, and each subject's exposure at the time of failure is compared with past exposures. One weakness of the case-crossover design is that time trends in exposure can cause bias. This is particularly true for environmental exposures such as air pollutants, which typically exhibit strong time trends.
Studies involving environmental exposures have certain advantages, however.
One is that exposure levels are not affected by failures. Thus we can determine the exposure that a subject would have had at any given time post-failure. The conditional case-crossover design takes advantage of this fact to produce estimates of risk that are insensitive to time trends in exposure. This method may prove useful for assessing risks of environmental exposures.