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?
closed as general reference by Hugo, aedia λ, ghoppe, James Waldby - jwpat7, Mitch Dec 5 '11 at 18:58
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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.