Selecting subpopulations for intervention

The effectiveness of a program designed to eliminate exposure to a risk factor may be enhanced by selecting specific subpopulations for the intervention effort. A second factor is called an intervention effect modifier of the first factor if the effect of the intervention. If an intervention effect modifier exists, a program's effectiveness may be enhanced by choosing a subpopulation for intervention on the basis of the distribution of that factor. We discuss the conditions in which intervention effect modification can occur when there is a third factor which influences risk. Additivity of incidence rates, the criterion when only two factors are involved, is neither necessary nor sufficient to rule out intervention effect modification in this situation. We also discuss the cost effectiveness of intervention strategies when the cost per successful intervention varies.