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Effectiveness means the capability of producing an effect. The word is sometimes used in a quantitative way, "being very or not much effective". However it does not inform on the direction (positive or negative) and the comparison to a standard of the given effect. efficacy, on the other hand, 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. Therefore, what is effective is not necessarily efficacious, and what is efficacious is not necessarily efficient.
An ordinary way to distinguish between effectiveness, efficay and efficiency:
- effectiveness: doing something
- efficacy: doing the "right" things
- efficiency: doing the things "right"
For the medical meaning of 'effectiveness', see discussion at efficacy.