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  • 1 a : producing a decided, decisive, or desired effect <an effective policy>
b : impressive, striking <a gold lamé fabric studded with effective…precious stones — Stanley Marcus>
  • 2 : ready for service or action <effective manpower>
  • 3 : actual <the need to increase effective demand for goods>
  • 4 : being in effect : operative <the tax becomes effective next year>
  • 5 of a rate of interest : equal to the rate of simple interest that yields the same amount when the interest is paid once at the end of the interest period as a quoted rate of interest does when calculated at compound interest over the same period


Effectiveness means the capability of producing an effect.

The word effective 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 among effectiveness, efficacy, and efficiency:

  • efficiency: doing things in the most economical way (good input to output ratio)
  • efficacy: getting things done, i.e. meeting targets
  • effectiveness: doing "right" things, i.e. setting right targets to achieve an overall goal (the effect)
  • (effectivity: mostly synonym to effectiveness; usage is rather rare)