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



efficacy (usually uncountable, plural efficacies)

  1. Ability to produce a desired effect under ideal testing conditions.
    • 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.
  3. Ability to produce a desired amount of a desired effect.


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