Although they look and sound similar, effectiveness, efficiency, and efficacy mean different things. So which one do you use, and when?
Effectiveness is the state of being adequate to accomplish a purpose or to produce the intended or expected result.
Efficiency is competency in performance, accomplishment of or ability to accomplish a job with minimum expenditure of time and effort.
And efficacy is the capacity for producing a desired result or effect. Ha, got you; this is actually interchangeable with effectiveness in most cases. It's the way of saying it that makes you sound smarter (because anyone can take an adjective and add "-ness" to the end to make a noun, but it takes really smart people to know the irregular version).
You can be effective without being efficient. I can cook a large, delicious, complex meal for dozens of people, which would be effective at feeding and satisfying my guests, but it would have taken me all day to prepare, and would therefore not be efficient.