efficacy vs. effectiveness

[serenissima]

I wondered what the difference was, so I looked it up.

Main Entry: ef·fi·ca·cy
Pronunciation: 'e-fi-k&-sE
Function: noun
Inflected Form(s): plural -cies
: the power to produce an effect

Main Entry: ef·fec·tive
Pronunciation: i-'fek-tiv, e-, E-
Function: adjective
1 a : producing a decided, decisive, or desired effect
1 b : IMPRESSIVE, STRIKING
2 : ready for service or action (effective manpower)
3 : ACTUAL (the need to increase effective demand for goods)
4 : being in effect : OPERATIVE (the tax becomes effective next year)
5 of a rate of interest : [snipped definition I'm not interested in]
- ef·fec·tive·ness noun

Hmm. Still not entirely sure.

Comments

[stryck.livejournal.com]

The big difference is that one is a noun and the other is an adjective.

Something is effective, therefore it has efficacy.

I believe effectiveness is a linguistic newcomer (relatively).

[403.livejournal.com]

Something with efficacy has the ability to produce an effect - not necessarily the desired effect.

[eredien]

This is really interesting...the difference is clearer in Russian, where there's a different verb for "know how to" and "can," such that the sentence "I know how to speak English" is not the same as "I can speak English." It's a difference between ability and possibility--which I think, is something that I didn't know English also had. How useful.