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- The important assumptions in a case crossover design include that the exposure should be brief, the time between exposure and event onset should be short and there is little carryover effect on the exposure. Therefore, this study design canít be used, for example, to evaluate an intervention which is anticipated to cause cumulative changes in the participants, or to study on the progression of a chronic disease. 

- Because of the above conditions, the results from a case crossover analysis only represent short-term risk but not cumulative risk, and relative risk but not absolute risk.