39

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?

Tamás
  • 951
  • Agree with Hugo, the difference is not really subtle at all. Check a dictionary. – ghoppe Dec 05 '11 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 Jul 05 '13 at 22:15
  • @Fixee I agree. The efficacy of this post far surpasses the intended efficiency of closing it. – monsto Aug 17 '18 at 15:33
  • Efficacy means getting things done Effectiveness means doing the right things Efficiency means doing things right –  Jan 20 '21 at 00:22
  • 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 Apr 01 '21 at 12:28

1 Answers1

38

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.

Mehper C. Palavuzlar
  • 35,654
Irene
  • 12,562
  • 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 Dec 05 '11 at 15:03
  • In pharmacology, actually, a clear distinction is made between efficacy and effectiveness (theory vs. practice), see e.g. http://books.google.de/books?id=4kZmAwAAQBAJ&pg=PA90 (p.90 of "A Dictionary of Epidemiology") for both terms. – anonymous Aug 26 '14 at 19:32
  • Efficacy means getting things done Effectiveness means doing the right things Efficiency means doing things right –  Jan 20 '21 at 00:22