MEASURES OF EFFECT BASED ON THE SUFFICIENT CAUSES MODEL .2. RISKS ANDRATES OF DISEASE-ASSOCIATED WITH A SINGLE PREVENTIVE AGENT

We considered a simple formulation of the sufficient causes model, in which a preventive agent exerts its effect by preventing a sufficient cause of the disease from occurring, while leaving another sufficient cause unaffected. In a group unexposed to the preventive agent, a case of the disease is caused by whichever of the two sufficient causes oc curs alone or first in the subject. Among exposed subjects, the preven tive agent prevents only the cases of disease in which the sufficient cause it blocks would have occurred alone, not the cases in which the other sufficient cause also occurs during the study period. The propor tion of subjects who would avoid the disease if exposed to the prevent ive agent is the risk difference. The risk difference varies over time , even when the rates of occurrence of the sufficient causes are const ant. It increases to a maximum and then declines, as the subjects who have avoided the disease because of the agent later contract the same disease because of exposure to the other sufficient cause. This maximu m and the time at which it occurs are readily computed from the incide nce rates of disease among exposed and unexposed subjects.