The quality or state of being efficacious; the power to produce a desired effect or intended result.
From efficacious (adjective) + -ness (noun suffix). This creates a parallel to efficacy, offering an alternative noun form emphasizing the quality itself rather than its measure.
English loves giving us multiple ways to say the same thing: 'efficacy,' 'efficaciousness,' and 'effectiveness' all mean roughly the same, but scientists often prefer the Latin-rooted terms!
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