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I usually use the word "effectiveness" in conversation, but sometimes I use the word "efficacy" then self-correct with "effectiveness" . Is there a practical difference between them?

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For all practical purposes, I don't think there's any difference in meaning between efficacy and effectiveness - or indeed efficaciousness, come to that.

My own feeling is that efficacy is somewhat stilted - pompous, even, despite the fact that it's a shorter word...maybe people like to use efficacy because it sounds more scientific. Possibly my perception is influenced by the fact that effectiveness has been the more common version for the past century...

Here a link that notes the difference: http://www.ahrq.gov/clinic/tp/efftrialtp.htm...Note "to distinguish effectiveness (pragmatic) from efficacy (explanatory) trials " Also, more at wikipedia.

However, based on past experience with hearing the word in scientific contexts, I googled efficacy and found that "In medicine, it is the ability of an intervention or drug to reproduce a desired effect in expert hands and under ideal circumstances." I guess that distinguishes it from effectiveness in practical real-world application. I know that one company I worked for conducted both "efficacy trials" and "field trials" for its products. enter image description here

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It rings true to me that in the specialied jargon of the pharmaceutical industry, effficacy might well be even the preferred term. Why don't you copy the whole of my answer in to yours, which I can then delete? I think between us we have it nailed (and you could use the points more! :) –  FumbleFingers Jan 5 '12 at 21:52
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How about I just downvote yours and you upvote mine, and we'll call it even? :) I think you nailed it on the pomposity factor, though...people like to use efficacy because it sounds more scientific. –  JeffSahol Jan 6 '12 at 0:20
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Well, one good answer is better than two half-good ones. Unless you ask me not to I might just copy mine into yours anyway (wonder what the site etiquette is on that! :) –  FumbleFingers Jan 6 '12 at 1:36
    
@FumbleFingers: having two almost identical answers from two different authors looks really...really suspect. At least there's a bit trail in the editing, but the first impression is that one of the authors is plagiarizing the other. I think I'd prefer each to stand alone. –  Mitch Jan 6 '12 at 14:06
    
I added his answer to mine, per his request....@FumbleFingers, can you delete now? –  JeffSahol Jan 6 '12 at 14:17
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To me, "efficacy" and "effectiveness" are essentially parallel to "utility" and "usefulness". The former connotes an idea of being used for the proper purpose while the latter simply indicates that it works. Using a screwdriver to put in a screw is both effective and efficacious. It has high utility and usefulness. Using the screwdriver to pry up a paint can would arguably do the same. But if you're using the same screwdriver as an improvised chisel, it is effective and it is useful, but that's not the intended purpose.

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The distinction between use and utilise has been covered before, but I don't think that has any bearing on your (to me, spurious) distinction between efficacious and effective. –  FumbleFingers Jan 5 '12 at 19:34
    
:) I honestly didn't know if anyone would share my impression. Connotations of words are always tricky because they've often picked up shades of meanings that have no real historical basis other than that of those involved in the conversation. –  Sean Duggan Jan 5 '12 at 20:47
    
Yeah - apologies for that (to me, snarky) (to me spurious) there. In general I thoroughly approve of people sensing fine distinctions between two word. Collectively, it's how we enrich our language over time, and individually it's a real buzz discovering that you're not alone in detecting some subtlety that hasn't yet made it into "common knowledge" and dictionaries. It's all about having your finger on the pulse. Even so, in this case I do think "efficacy" is just a somewhat "dated" word, but that might be because "efficacious" is effectively a cliched archaism nowadays. –  FumbleFingers Jan 5 '12 at 21:44
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In general usage, effectiveness and efficacy have the same meaning. In medical jargon there is a subtle, but important difference. From Wikipedia:

In medicine, effectiveness relates to how well a treatment works in practice, as opposed to efficacy, which measures how well it works in clinical trials or laboratory studies.

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Based on their definitions, and this thesaurus entry, I'd say they are synonyms, and the difference is purely stylistic word choice/personal preference.

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-1 for differency and purley –  jwpat7 Jan 5 '12 at 18:41
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@jwpat7 For typos? Really? –  Jim Jan 5 '12 at 19:11
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@jwpat7, you should have said "for typoes? rly?" –  JeffSahol Jan 5 '12 at 20:56
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