The efficacy-effectiveness gap

Abstract The terms efficacy and effectiveness are commonly used in medical treatment research, for example drugs. Efficacy refers to the effect a drug could have under ideal circumstance. Effectiveness, instead refers to the effects a drug has in daily practice. The difference between the two is referred to as the efficacy-effectiveness gap. In this chapter several intrinsic and extrinsic factors that influence this gap are discussed. Thinking in terms of the efficacy-effectiveness gap aids in articulating the research objectives and aligning the trial design with these objectives.