noun, plural efficacies.
capacity for producing a desired result or effect; effectiveness: a remedy of great efficacy.

1520–30; < Latin efficācia, equivalent to efficāc- (see efficacious) + -ia -y3

nonefficacy, noun
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
efficacy (ˈɛfɪkəsɪ)
the quality of being successful in producing an intended result; effectiveness

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1520s, from L. efficacia, from efficax (gen. efficacis) "effective," from efficere "work out, accomplish" (see effect).
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
The efficacy of research is by no means to be demeaned.
Some dermatologists have raised questions about the efficacy of the products
  and their claims.
The manufacturer also touts its efficacy among the hearing impaired or very old.
As life could not exist without service, neither can life survive without the
  efficacy of a beneficial service.
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