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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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.