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- In summary, case crossover design is used when a brief exposure causes a transient change in risk of an acute onset event. The event can be the onset of type I diabetes, a motor vehicle accident, or an injury. The �exposure� can be another disease or certain activity, such as having an infectious disease, conducting a certain unusual task, or using a certain device. There is a hazard period associated with the event onset and the exposure information in the hazard period is compared with the control data from a selected control period. 

- In the next few slides I will formally introduce hazard period, effect period and control data.