Differential misclassification of exposure in case-crossover studies.

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.