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These two words are very similar in meaning, and yet they are not the same.
I have often heard students use these two words interchangeably.
Efficacy means the ability to produce the desired result.
Effectiveness means the degree to which something is successful in producing a desired result.
Sounds the same, right?
Well, "efficacy" is used to describe the result of a clinical trial.
For example, a vaccine has a 95% efficacy. It means in a trial, where the environment is controlled, the chance that the vaccine works is 95%.
However, in the real world, there are many factors affecting the effectiveness of a vaccine.
For example, the person taking the vaccine shot may have some health problems. Age is also a factor.
There is a very simple way to remember the difference in meaning of these two words.
Efficacy - in a trial
Effectiveness - in the real world
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