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"effectiveness" is also used as a crossword clue. If that's what you saw, the answer is often efficacy (8 letters).
Definitions
capacity to produce strong physiological or chemical effects
Word origin
From effective + -ness.
Used in a sentence
“The effectiveness of the drug was well established.”
“The problem with these precedents is that the Security Council did not authorize intervention in Syria, and an intervention there will thus harm, not help, the Council’s credibility and effectiveness.”
“He questioned the effectiveness of the treatment.”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer1 curated clues
01“Ability to work”13 letters
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