The three terms are sometimes used interchangeably, however, each of them should be used in certain events. The following definitions will address how these terms differ, and when should each be used:
Efficacy: The benefit of using a technology, program or intervention to treat a particular problem under ideal conditions—for example, in the context of research in a laboratory or a rigorous protocol for a randomized clinical trial.
Alternative definition: Efficacy is the ability to produce a desired amount of the desired effect, or success in achieving a given goal. Contrary to efficiency, the focus of efficacy is the achievement as such, not the resources spent in achieving the desired effect.
Effectiveness: The benefit of using a technology, program or intervention to address a specific problem under general or routine conditions, rather than under controlled conditions, for example, by a physician in a hospital or by a patient at home.
Alternative definition: Effectiveness is doing “the right” things, i.e. setting right targets to achieve an overall goal (the effect).
Efficiency: The ability to obtain the maximum possible benefit, given the resources available.
Alternative definition: Efficiency is doing things in the most economical way (good input to output ratio).
Note: It is often the simple ratio between the outcome obtained and the resources used.