Here the myocardial infarction study by Mittleman MA (1993) is used to illustrate the types of control data we can use in a case crossover study. In the study they investigated the effect of heavy physical exertion on the onset of MI and whether the risk of MI can be modified by regular physical exertion. Thus here, the "event" is the onset of MI, and the "exposure" is heavy physical exertion. The effect of physical exertion is assumed to last one hour, and the hazard period is one hour before the onset of MI.