Methodological Assessment of Case- Crossover Studies

The case-crossover design was developed in the early 1990s to study the effects of transient, short-term exposures on the risk of acute events such as myocardial infarction. To estimate relative risk, the exposure frequency during a period just before outcome onset (hazard period) is compared with exposure frequency during control time(s) in that person rather than in a control. One or more "control times" are supplied by each of the cases themselves to control for confounding by constant characteristics and self-confounding between the trigger's acute and chronic effects. In the analysis of case-crossover studies, exposure frequency in the hazard period is compared with the control period or the individual's usual frequency of exposure. The design has been used frequently for heart diseases, injuries and air pollution epidemiology. This review article looks at published case-crossover studies and is intended to help the reader gain a better understanding of the strengths and limitations of the case-crossover design in studying the epidemiology of injuries and air pollution.