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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