Efficacy is the ability to produce a desired amount of the desired effect or success in achieving a given goal. In healthcare, it is the capacity for beneficial change (or therapeutic effect) of a given intervention (for example a drug, medical device, surgical procedure, or a public health intervention) under ideal or controlled conditions.
Context: Clinical trials
Effectiveness is setting the right targets to achieve an overall goal (the effect). It is the extent to which planned outcomes, goals, or objectives are achieved as a result of an activity, intervention, or initiative intended to achieve the desired effect, under ordinary circumstances. In healthcare, it is the extent to which a drug achieves its intended effect in the usual clinical setting.
Context: Pragmatic settings, observational studies
Efficiency is the ratio of the output to the inputs of any system (good input-to-output ratio). An efficient system or person achieves higher levels of performance (outcome, output) relative to the inputs (resources, time, money) consumed.
Context: Cost-effectiveness study, for example, measuring nursing hours, bed days, and days supply of drugs.