An effect parameter, indicating the proportion of diseased persons (cases) which can be attributed to exposure. This proportion (AP^.) is obtained by dividing the rate difference by the rate among the exposed: APc= (I1 – I0)/I1. This is also a parameter for the proportion of cases in the total population which can be attributed to exposure. The total population can be divided in a proportion unexposed individuals (P0) and a proportion of exposed individuals (P1). The incidence rate in the total population (I,) can be calculated from I1 = P0l0 + Pili. The attributable proportion among the total population (AP1) is defined as AP1 = (I1 – I0)/I1.