The terms efficacy and effectiveness are frequency used in health care. The first term, efficacy is the ability of a treatment to work in ideal or controlled conditions, whereas effectiveness is the ability of a treatment to work in real-world settings.

Efficiency, on the other hand, is a term referred to the ratio of the output to the inputs of any system. It is commonly used term in an economical aspect (i.e. cost-effectiveness of the intervention and an outcome) . To give an example, a high efficacy drug in trial might be high, yet the cost spent on the treatment is no superior than the conventional treatment. Thus, it is not considered efficient, or not cost-effectiveness.