Late last week I was in a meeting with a chappie from the International Organization for Standardization, talking about the role of the research group I belong to and explaining how we measure out performance. This sort of thing is typical of any company that needs to maintain its ISO status  and having lists of procedures, processes, and parametrics is de rigueur for the whole shebang.
In the course of the discussion, I happened to talk about the challenge of measuring the efficacy of a department whose purpose is to generate speculative ideas, 80% of which are likely to be unfeasible. The examiner stopped me and asked me to repeat the word, which I did, and my colleague also offered a “translation” by saying “effectiveness.” That did the trick and chalked it up to my being an Englishman who is still struggling to learn American. 
But being me, I jotted the words down in my ever-present notebook with a few to investigating whether the efficacy/effectiveness was, indeed, a transatlantic difference.
Of course, in this age of Evidence-Based Practice, the call for measures how much effect therapy has on a client means that it’s common to talk about the “efficacy of treatment” or the “effectiveness of an approach.” Or is it? Do we say “efficacy” or “effectiveness?” Is there, in fact, a difference?
Well, the first thing I often do with questions like this is to use the Google search engine and get a Ghit measure. “Ghit” is short for “Google Hit” and appears in a search as a number under the search bar.  Here’s what comes up for efficacy and effectiveness:
Efficacy: 17,100,000 ghits
Effectiveness: 179,000,000 ghits
Whoa! Quite a difference there, by a factor of ten. Just to corroborate the difference, I did a Bhit count and a Yhit count (Bing Hits and Yahoo Hits, if you weren’t sure).
Efficacy: 52,400,000 bhits and 52,600,00 Yhits
Effectiveness: 143,000,000 Bhits and 139,000,000 Yhits
So not ten times larger for effectiveness but still significantly more popular. But what about the notion that it’s a UK/US thing? After all, it is possible that the high ghit count is masking it – after all, the percentages will always skew in favor of the US when it comes to number of speakers.
This is when I turn to my trusty friend, the BYU-Corpus site, where we can play with the Corpus of Contemporary American to check on how a word is used in the US, and also the British National Corpus to get a UK perspective. I did this for my previous post on the use of have versus take in relation to bathing – and this turned out to be most definitely a US/UK distinction. Here’s what we see;
Oh bugger! It doesn’t look like a BrE versus AmE difference after all. There is a 10% variation between the two but I’m pretty sure it’s not statistically significant. My choice to use efficacy puts me in the minority in both the States and the Isles.
Desperate for some validation, I dug a little deeper by looking at some historical data. Maybe I’m just old and the incidence of the words has changed since I was a lad. The British National Corpus isn’t much help as it only covers the period from the 1980’s through to 1993, and I want to see older data than that.
The Oxford English Dictionary is a good source for historical information on word meaning, so I went to the bookshelf and did a little more research.
Efficacy as a noun dates from 1527 and is defined as the “(p)ower or capacity to produce effects.” It’s derived from the earlier Latin efficere meaning “to accomplish.” Its meaning hasn’t really changed since then and so we can call it a 16th century word – old enough.
Effectiveness as a noun is a little younger, with the OED identifying a first appearance in 1607, almost a hundred years after efficacy. It has a similar definition of, “(t)he quality of being effective.” Not surprisingly, it, too, can be traced back to the same Latin root as efficacy, efficere. However, it is a 17th century word so I can take some comfort (perhaps) in arguing that my use of efficacy is more “traditional.”
However, we can see something much more interesting if we take a peek at the Corpus of Historical American, which cover the period 1810-2009, and that certainly goes back further than my birth!
Here’s the chart of the behavior of the word efficacy since 1810:
Even before you click on the image to enlarge it, it’s clear that efficacy has been in a slow decline for decades. There’s been a modest upswing since the 1950’s but it’s nowhere near its glory days. So the inevitable question is, what has pushed it aside?
Well, well, well, what a surprise! The usurper turns out to have been no more than the Pretender to the Throne, effectiveness! From out of the shadows, the word has slowly increased its popularity to the point that it now hogs the limelight and commands center stage. Alas, poor efficacy, I knew it, Horatio.
The story might end there, with my claiming to be simply the sort of dude who uses older words, and who also is victim to the invisible hand of lexical change that can overturn the fortunes of synonyms. But there is something else: Although for most of the world, efficacy and effectiveness are synonymous (and dictionaries typically say that) there is a field in which they are not synonymous: the Clinical World.
Ah. but that’s a story for another day…
 For some time, I took pleasure in pointing out that the “International Organization for Standards” was clearly guilty of failing to notice that the acronym should be IOS and ISO. Alas, my mistake was to assume the ISO was an acronym, when in fact, it allegedly isn’t! The organization say that it’s derived from the Greek word isos, meaning “equal” and that they did this so they wouldn’t have to use different acronyms in different countries based on the languages. For example, in France it would be Organization Internationale de Normalization (OIN), so ISO is international.
 When folks ask me if I speak more than one language, I say I’m bilingual and can speak both English AND American. One of the delights of being an Englishman Abroad is that not only have I had the chance to be immersed in the UK’s melange of dialects and accents for the first 30-something years of life but now I get to go through it all over again with the different flavors and recipes of American English. I’m comfortable with Fall, happy to spell tyres as tires, and say “to-MAY-toe” and not “to-MAH-toe.”
 The accuracy of using ghits as a measure of word use is always open to question but as a quick and dirty metric it’s used by linguists who want to get a feel for how the world of words is playing out. Arnold Zwicky used them in a recent blog about the prefix “telephon-” and Geoff Pullum has them in a post on “Assholocracy,” so I think I’m in pretty good company.