The case-crossover design has become an accepted method for investigating acute health consequences of transient exposures, or “triggers”. This design, originally presented in 1991 by Maclure,¹ can be thought of as a variant of the conventional case-control study with individual (pairwise) matching. The principal difference is that a case-crossover study only includes cases, where each serves as his own referent. This feature has a compelling methodological appeal in that it allows for efficient control of potential confounders that are fixed (for example, genetic traits) or change minimally or slowly over time (for example, socioeconomic status), and are difficult or impractical to measure. Who better to provide control for confounding than the case himself?

To date, the case-crossover design has been applied most often to investigate environmental exposure effects in studies of air pollution, as an alternative to time series analyses. The paper by Schwartz² in this issue exemplifies this application, and also illustrates the utility of the case-crossover method for controlling for seasonal and long term air pollution time trends. There have also been several applications to …