different between effective vs efficacy

English

Etymology

From French effectif, from Latin effect?vus (“productive; effective”), from effici? (“I make; I bring about”).

Pronunciation

• (weak vowel distinction) IPA^(key): /??f?kt?v/
• (weak vowel merger) IPA^(key): /??f?kt?v/
• Rhymes: -?kt?v

Adjective

effective (comparative more effective, superlative most effective)

1. Having the power to produce a required effect or effects.

   Synonym: efficacious

2. Producing a decided or decisive effect.
   • 1650, Jeremy Taylor, The Rule and Exercises of Holy Living

     Whosoever is an effective, real cause of doing his neighbour wrong, is criminal.

3. Efficient, serviceable, or operative, available for useful work.

4. Actually in effect.

5. (geometry, of a cycle or divisor) Having no negative coefficients.
6. (physics, for any effective theory) approximate; Not describing the fundamental dynamic changes in some system as they happen.

Usage notes

Webster's Revised Unabridged Dictionary from 1913 still lists efficient and effective as synonyms, but all major dictionaries now show that these words now only have different meanings in careful use. Use of both for the other meaning is however widespread enough that Longman's Exam Dictionary, for example, finds it necessary to proscribe the use of one for the other with several examples at each entry and provides the following summary:

• efficient = working quickly and without waste
• effective = having the desired effect

Related terms

Translations

Noun

effective (plural effectives)

1. (military) a soldier fit for duty
   • 1876, Dabney Herndon Maury, Southern Historical Society Papers: Volume 2, Number 4, Recollections of the Elkhorn Campaign:

     The Army of the West reached Corinth sometime after the battle of Shiloh. We were 15,000 effectives, and brought Beauregard's effective force up to 45,000 men.

French

Pronunciation

• IPA^(key): /e.f?k.tiv/
• Homophone: effectives

Adjective

effective

1. feminine singular of effectif

Latin

Adjective

effect?ve

1. vocative masculine singular of effect?vus

English

Etymology

From Old French efficace, from Late Latin effic?cia (“efficacy”), from effic?x (“efficacious”); see efficacious.

Pronunciation

• IPA^(key): /??f.?.k?.si/

Noun

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

Synonyms

• efficacity

Related terms

Translations

Further reading

• efficacy in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913.
• efficacy in The Century Dictionary, New York, N.Y.: The Century Co., 1911.