effectivity vs efficacy

effectivity and efficacy both are nouns.

Word	Noun	Adjective	Verb	Adverb
effectivity	Yes	No	No	No
efficacy	Yes	No	No	No

As nouns, efficacy is a hyponym of effectivity; that is, efficacy is a word with a more specific, narrower meaning than effectivity:

effectivity: power to be effective; the quality of being able to bring about an effect
efficacy: capacity or power to produce a desired effect

Other hyponyms of effectivity include incisiveness, trenchancy, efficaciousness.

effectivity (noun)	efficacy (noun)
power to be effective; the quality of being able to bring about an effect	capacity or power to produce a desired effect