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I feel that there is a subtle difference in meaning between 'efficacy' and 'efficiency', but I couldn't find any authoritative sources that could help me confirm or refute this. Is there any difference between the two words, especially in scientific context?

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  • Agree with Hugo, the difference is not really subtle at all. Check a dictionary.
    – ghoppe
    Commented Dec 5, 2011 at 15:10
  • 12
    Googling for the difference between these words lands you here at the top result. And this gives a concise description of the difference (rather than requiring a reading of two definitions and then a mental comparison), so thanks to the OP for asking.
    – Fixee
    Commented Jul 5, 2013 at 22:15
  • @Fixee I agree. The efficacy of this post far surpasses the intended efficiency of closing it.
    – monsto
    Commented Aug 17, 2018 at 15:33
  • Efficacy means getting things done Effectiveness means doing the right things Efficiency means doing things right
    – user400043
    Commented Jan 20, 2021 at 0:22
  • The question was about finding authoritative sources. I still think this is relevant: is the difference real, or something people keep parroting on the web?
    – ymajoros
    Commented Apr 1, 2021 at 12:28

1 Answer 1

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Efficacy means effectiveness, i.e. the ability of something, e.g. a medicine, to produce the results you want.

Efficiency means ability to do something well without a waste of time or money, e.g. I was impressed by her efficiency in this project.

For a more technical context, I found the following additional definitions of efficiency:

a)The ratio of the effective or useful output to the total input in any system. b) The ratio of the energy delivered by a machine to the energy supplied for its operation.

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  • 14
    Agreed - Efficacy is about the results, whereas Efficiency is about waste when achieving the results. Something could be said to have good efficacy but poor efficiency, if it was effective but comparatively costly or wasteful. Likewise, something could be considered to show good efficiency but poor efficacy, if it only is moderately effective, but isn't particularly wasteful in the process.
    – CJM
    Commented Dec 5, 2011 at 15:03
  • In pharmacology, actually, a clear distinction is made between efficacy and effectiveness (theory vs. practice), see e.g. books.google.de/books?id=4kZmAwAAQBAJ&pg=PA90 (p.90 of "A Dictionary of Epidemiology") for both terms.
    – anonymous
    Commented Aug 26, 2014 at 19:32
  • Efficacy means getting things done Effectiveness means doing the right things Efficiency means doing things right
    – user400043
    Commented Jan 20, 2021 at 0:22

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