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A0: Regardless of exposure, these people will become cases within time t. Exposure has no impact on incidence time A1: Regardless of exposure, these people will become cases within time t. Exposure influences incidence time. A2: In absence of exposure, these people will not become cases within time t. Excess fraction = A2/A+ Etiologic fraction = (A1 + A2)/A+ A+ = A0 + A1 + A2