efficacy

Etymology

From Old French efficace, from Late Latin efficacia (“efficacy”), from Latin efficax (“efficacious”); see efficacious.

Pronunciation

• IPA^(key): /ˈɛf.ɪ.kə.si/

Noun

efficacy (usually uncountable, plural efficacies)

1. Ability to produce a desired effect under ideal testing conditions.
   • 1642, Sir Thomas Browne, “Religio Medici [The Religion of a Doctor]”, in The Works Of the Learned Sr Thomas Brown, Kt., London: Tho. Basset et al., published 1686, page 15:

     […] and this hath even made me ſuſpect the efficacy of reliques, to examine the bones, queſtion the habits and appurtenances of Saints, and even of Chriſt himſelf.

   • 2005, Flay et al. Standards of Evidence: Criteria for Efficacy, Effectiveness and Dissemination DOI: 10.1007/s11121-005-5553-y

     Efficacy refers to the beneficial effects of a program or policy under optimal conditions of delivery, whereas effectiveness refers to effects of a program or policy under more real-world conditions.

2. Degree of ability to produce a desired effect.
   • 1996, Moskovich, Patent application PCT/US1996/003658

     Toothbrush with improved efficacy

   • Wikipedia entry for "vaccine efficacy", as of 2016

     Vaccine efficacy is the percentage reduction of disease in a vaccinated group of people compared to an unvaccinated group, using the most favorable conditions.