I have a confession: I’m a compulsive looker-upper. So today, even though I was pretty sure it was OK to use the word “efficacy” in the sentence I was writing, I checked anyway. Sure enough, in most contexts it’s an appropriate synonym for “effectiveness.” (I liked efficacy in this case, because I was talking about something that had been statistically proven, and I think it sounds more “science-y.” How’s that for a peek inside a writer’s brain?)
But I also like looking things up because I almost always learn something. Today’s lesson was that, in medical parlance, “efficacy” and “effectiveness” mean different things, and it’s a nuance that’s quite significant. Efficacy is a narrower definition that means how well something works in an ideal or controlled setting, such as a clinical trial. Effectiveness describes how well it works under “real-world” conditions. Effectiveness, for example, takes into consideration how easy a drug is to use, and potential side effects, whereas efficacy measures only how well it produces the desired result.
The moral: In academia, particularly in science or medicine-based fields, you probably want to be careful about interchanging these words. But for the layman, they are essentially synonymous.