How do assessments of the efficacy of an intervention differ from assessments of an intervention’s effectiveness?

What will be an ideal response?

One distinction often made in impact evaluation is between assessments made of the efficacy of an intervention and those made of its effectiveness. In this context, assessments of efficacy test an intervention under favorable circumstances, often in a relatively small study at a single site. These studies are frequently conducted by the developer of an intervention as an early “proof of concept” step for determining if it has promise for affecting the targeted outcomes. Assessments of effectiveness, in contrast, are oriented toward estimating the intervention effects for a fully deployed program implemented at scale and delivered as routine practice to typical members of the target population. Most ongoing programs studied by impact evaluators are of this sort. The circumstances of service delivery may be less than optimal and participants will be typical for the program context, whether especially well matched to the program. The program developer or associated personnel may have provided training to the service delivery personnel, but they are not themselves part of the team that delivers the program.