ef·fi·ca·cy | \ ˈe-fi-kə-sē \
plural efficacies

Definition of efficacy 

: the power to produce an effect

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Examples of efficacy in a Sentence

In the planner's view, Rumsfeld had two goals: to demonstrate the efficacy of precision bombing and to "do the war on the cheap." —Seymour M. Hersh, New Yorker, 7 Apr. 2003 Vaccines exist, but their efficacy against aerosolized plague is unknown. —Sharon Begley et al., Newsweek, 8 Oct. 2001 efficacy does not have to be demonstrated before homeopathic products are marketed. —Alison Abbott et al., Nature, 26 Sept. 1996 questioned the efficacy of the alarms in actually preventing auto theft
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Recent Examples on the Web

The researchers ran computer simulations on a hypothetical cohort of over 360,000 women to test the efficacy and costs of risk-stratified screening. Orly Nadell Farber, STAT, "New approach to breast cancer screening — tailoring guidelines for each patient — may save lives and money, study says," 5 July 2018 In Connecticut, Yale New Haven Hospital is partnering with Epic to establish a capacity command center, using real-time data and electronic health records to improve efficacy and quality of care. Naseem S. Miller, OrlandoSentinel.com, "Florida Hospital building state-of-the-art command center," 22 June 2018 But the results of that China-U.S. pact have left many a little jaded about the efficacy of diplomacy on this issue. Washington Post, "Summit does not deal with North Korea’s hacking," 17 June 2018 While there are many disagreements about how to do this, spreading false narratives about efficacy just keep Georgia standing still both literally and figuratively. Doug Turnbull, ajc, "Gridlock Guy: Don’t just believe campaign half-truths about traffic," 18 May 2018 After the recent heists, collectors and purveyors of jewelry alike have wondered about the efficacy of modern security systems. Eric Konigsberg, Town & Country, "Old School Jewelry Heists Are on the Rise—And Celebrities Are the Target," 12 Apr. 2018 Research about the efficacy of nicotine replacement therapy using tools such as e-cigarettes and nicotine gum is relatively inconclusive. Jamie Ducharme, Time, "Teens Are 'Juuling' At School. Here's What That Means," 27 Mar. 2018 Another is Labdoor, a startup founded in 2012 in San Francisco, which ranks various supplements on a 100-point scale for label accuracy, purity, nutritional value, efficacy, and safety. Peter Andrey Smith, Outside Online, "Don't Trust the Label on Your Supplements," 5 July 2018 Stimulant medication is the first line of treatment for adult ADHD and has the greatest margins of efficacy with only mild tolerable side effects. Rene Hernandez-cardenache, Psy.d., M.s. Ed., miamiherald, "A focus on distraction: A closer look at adult ADHD," 2 July 2018

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'efficacy.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of efficacy

13th century, in the meaning defined above

History and Etymology for efficacy

see efficacious

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Statistics for efficacy

Last Updated

11 Sep 2018

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Time Traveler for efficacy

The first known use of efficacy was in the 13th century

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English Language Learners Definition of efficacy

: the power to produce a desired result or effect

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