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The aim of this study is to examine 2 types of differential misclassification of exposure in case-crossover studies. The first is the outcome-dependent misclassification of exposure, meaning that if an event has occurred, it could affect the reporting of exposure. The second is differential misclassification of exposure as a result of fading memory over time, which arises if the length of the recall period for case and control windows differs. We use empirical data from a case-crossover study of triggers of attacks in Ménière's disease. The study applied the matched-pair interval control window sampling approach. We examined misclassification in relation to 2 different types of exposures: emotional stress and salty food intake. The study covered repeated events reported by the same patients and involved the sampling of many control windows. Because some of these windows were related to case events and some unrelated, we were able to conduct both case-crossover and control-crossover analyses. Although this group of Ménière patients are well aware of their disease, and many of them have definite ideas regarding what triggers attacks, neither outcome-dependent misclassification nor differential misclassification of exposure resulting from fading memory over time seemed to be a major problem.

The association between ambient air pollution exposure and hospitalization for cardiovascular events has been reported in several studies with conflicting results. A case-crossover design was used to investigate the effects of air pollution in 660 first-time myocardial infarction cases in Stockholm in 1993-1994, interviewed shortly after diagnosis using a standard protocol. Air pollution data came from central urban background monitors. No associations were observed between the risk for onset of myocardial infarction and two-hour or 24-hour air pollution exposure. No evidence of susceptible subgroups was found. This study provides no support that moderately elevated air pollution levels trigger first-time myocardial infarction.