The sufficient causes model of disease occurrence leads to a specific conception of how the risk of disease resulting from exposure to an agent combines with background risks. From this conception, one can derive a method for quantifying in populations the respective effects of the agent and background, using either rates or risks. This method differs from the usual difference or ratio measures of effect by taking into account the probability that the sufficient cause of disease involving the agent of interest and that not involving it will both occur during the observation period. The method leads to: (1) measures of the risks or rates of completion of sufficient causes involving or not involving the agent of interest; (2) a measure of the proportion of cases preventable by removing or blocking the agent, based on observed risks of disease. This proportion varies as a function of the duration of exposure; (3) a measure of the proportion of cases caused by the agent, based on observed rates of disease. This proportion is constant over time, if the rates are; (4) a causal interpretation of a constant rate ratio, when the rates vary over time.