efficacy

[ ef-i-kuh-see ]

noun

, plural ef·fi·ca·cies.

1. the capacity for producing a desired result or effect:

   Short, frequent periods of practice were shown to have greater efficacy than longer and less frequent ones.

2. a measure of the success of a vaccine or other pharmaceutical when used in the controlled environment of a clinical trial, as opposed to in the real world. Compare effectiveness ( def 3 ).

efficacy

/ ˈɛfɪkəsɪ /

noun

1. the quality of being successful in producing an intended result; effectiveness

Other Words From

• non·ef·fi·ca·cy noun

Word History and Origins

Origin of efficacy¹

First recorded in 1520–30; from Latin efficācia, from efficāc- (stem of efficāx ) “effective, effectual” + -ia, noun suffix; efficacious ( def ), -y ³( def )

Example Sentences

Regardless of what the Trump administration does, the efficacy of ballot measures may be limited in states under Republican control.

From Slate

It also directed that a committee be set up to look into the feasibility of moving power lines and the efficacy of bird diverters - devices that have reflectors and are attached to power cables to alert birds about their presence.

From BBC

While most experts agree that social media platforms can harm the mental health of adolescents, many are split over the efficacy of trying to outlaw them all together.

From BBC

But even back in 2012, there were those who realized that the fact-checkers’ efficacy relied on the public’s willingness to accept their authority.

From Slate

Evidence for canvassing’s efficacy is vexed, however: social scientists have shown the practice of going door to door to have near-zero persuasive power, and, unless the message is highly personalized, an uncertain effect on turnout.

From Salon

Related Words

• adequacy
• competence
• effectiveness
• potency
• virtue