How is IQ caluated at the lower ends of the scale?

We all know that it’s easy to diganose someone with mild or moderate mental retardation. But how do they calcuate IQ when someone has severe or profound mental retardation.
What does an IQ of 15 mean? What does an IQ of 5 mean?

Profound mental retardation is defined as an IQ below 20-25. I’m sure someone more expert than I will be along shortly, but I would be surprised to learn that there were significant distinctions below that point.

Well, the lowest measurable IQ available in the Wechsler Intelligence Scale for Children - Fourth Edition is 40, which falls at the lower end of the lower end of the moderate range of mental deficiency. Anything lower would have to be estimated/extrapolated, but the value of doing that is dubious, since you need to deal with the functional level of these individuals, not predict what they may be able to do by a score on paper.

BTW, the scale also tops out at 160. So when you hear folks (I think I’ve seen it here) brag about their IQ’s being above that level, you might be skeptical. They’ve either had their IQ’s extrapolated (possible, but doubtful), taken some type of quacky internet test, or are pulling the score out of their arse.

The Wechlser series is the most widely used in clinical/school practice, followed by the Stanford-Binet Scales.

Remember that I.Q. scores mean nothing except as a measurement of a place on a normal curve. Just as scores of 115, 130, 145, and 160 mean that someone is one, two, three, or four standard deviations above the mean (which is 100) , scores of 85, 70, 55, and 40 mean that someone is one, two, three, or four standard deviations below the mean. To assign someone a place of four standard deviations above the mean (which means that only one person in 31,000 gets that score), you would have to give the test to about 100,000 people and say that just the top three people on the test are at least four standard deviations away from the mean and thus have I.Q.'s of at least 160. Assigning I.Q.'s above that level is a very dubious proposition.

It’s harder to establish what an I.Q. of 40 means. Such a person can’t read, so you would have to use a special test. You generally have a list of simple tasks that any person of average intelligence can do and check how many of them the person can do.

The IQ is what its name implies–a quotient. Can the IQs of severely mentally disabled be derived by comparison with those who are not so badly handicapped? For example, a person with an IQ of 60 can usually speak normally if simply, may be able to write a little, or perform other tasks involving pencil and paper. Presumably there is a wide array of nonverbal tests which they can undergo as well; being able to do them would be, so to speak, “normal” for an adult with a 50 IQ. Could people more profoundly disabled be tested by having them attempt those tasks, and compare their performance to the 50-IQ benchmark at given ages, thus deriving the mental/chronological age quotient?

Incidentally, an I.Q. of 5 is as impossible as an I.Q. of 195. There haven’t been enough people in the history of the world to put someone that far out on the normal curve in either direction.

The I.Q. is no longer a quotient. That’s an old-fashioned definition of it. It’s now a place on a normal curve. That is, it tells you how many standard deviations you are from the mean, using 100 as the mean and 15 points for each standard deviation.

No comprehensive IQ test I’ve ever used required the subject to read. Vocabulary terms are sometimes made available to the subject, but always are read by the examiner.

Individuals in the 40 IQ range are difficult to assess because they can do so little of the test. Maybe older folks can mimic the responses of the examiner and get some credit on some tests, but on the whole they can do very little of what the test asks for.

Again, with these indivduals you’re dealing with a restricted range of language development and learned behavior.

Why is that “doubtful?” It happened to me. I got tested multiple times when I was a kid because my parents and teachers thought I was some kind of boy genius or something. I went to real shrinks, and took the real tests (there was no internet back then), and was told I went off the chart, (which, I guess is why they tested me more than once…I was tested three times in total) but the second shrink said it was probably around 165 (the first one wouldn’t hazard a guess). I don’t think my experience was rare.

That estimated score was the bane of my existence in high school. My parents and teachers expected me to be the next Stephen Hawking, but I was lazy, undisciplined, pot smoking underachiever, despite my “accelerated” curriculum. If I’d just tested out as an ordinary idiot, I wouldn’t have had to feel so guilty about letting people down all the time.

Ever seen Forest Gump?

I’m not saying it’s impossible, just extremely rare.

I’ve NEVER seen any reported IQ that high in any clinical psychological report I’ve read.

You mean you’ve never seen an estimated score over 160, or you’ve never seen a report of somebody topping out the Wechsler? I don’t know what to tell you, but they said it happened to me. I think that’s why they kept making me do it over again. It was my impression that this was unusual, but not so rare that no one would ever see it. Maybe I’m just naive enough to believe what other people have told me about their own experiences, but I’m certain that at least some of those people were brighter than me.

Do different scales have different endpoints?

Either. For someone to get every possible item correct on the test is pretty inconceivable to me, but I’m pretty average.

I have no reason to doubt what you are saying. If it is true, consider your brain power representative of 1 in 100,000.

Probably. But the highest ceiling I know of now is 160.

Definitely. At both ends. They also mean different things at various intermediate points. The only place all IQ scores mean the same thing is at 100 – dead average. An IQ score by itself is of limited usefulness unless you also know the test it’s based on.

Misleading numbers can also result from continuous re-testing. Or from extrapolating. The incredibly high numbers that are sometimes reported (“smartest woman in the world”) are the result of one or both of those.

But when the population of civilization tops 76 billion people, then the dumbest person alive will actually have negative IQ.

People at the very lowest end of the scale are frequently assessed using measurements of adaptive living skills, which are more useful in providing intervention and treatment than an I.Q.

Diogenes the Cynic, unless you’re old enough that you took I.Q. tests back before the scoring was converted from the old quotient definition to the modern standard deviation definition, no test could have measured your I.Q. as 165. None of them measure a score higher than 160. If what you say is correct, I suspect that what happened with the psychologist who claimed that your I.Q. was 165 was that he looked at the results of the two (or three) I.Q. tests you’d already taken, each of which gave your score as 160, and decided that you were probably even a little bit better than someone who just topped out on the test. He made an arbitrary guess that your I.Q. was about 165.

Could you tell us a little more about your life experiences since then? What did your parents try to do with someone who had a very high I.Q. but didn’t seem to be working anywhere close to his potential? What did your schools and colleges do? What have the jobs you’ve worked at been like?

OK, here’s the thing: I have no idea what scales have I been measured on. I was never told. I could probably find out for the only test for which I got a number, since it was Mensa Spain’s “official” test at the time. I don’t even know whether the “psychological” tests at school ever involved IQ tests; they probably did, but my parents put as much value on that as on the “professional interests” test (i.e., they used it as toilet paper).

So as you say, maybe the people who’ve been given numbers above 160 were given a number taken out of the tester’s left elbow (in which case the one who was lying, as in giving misleading information, and absolutely unprofessional, was the tester), or maybe they were tested under one of those scales which never survived more than a year.

It was around 1979 or 1980, so if the scoring has changed that might exolain something. My impression was never that my score was freakish, just unusual.

I wasn’t told that I had scored 165 on the test, but that I had gone of the chart at 160, and the second shrink who tested me said he thought it was probably around 165 – He didn’t act like it was an Oh my God, Marilyn Vos Savant level or anything, but just a “really smart” kid who could probably stand to skip a grade or two in school.

It first started in 1st grade, when some teachers urged them to test me and maybe move me up a grade. I think they did test me then, but I don’t remember much about it, and they decided nt to move me up because they wanted me to socialize normally. Around 6th grade, they started getting questions from teachers again, and then a year or two later, my parents started doing the serious testing. They tested my bothers too, and both of them tested really high as well, but I was the only one who seemed to get all the attention. Eventually, my parents and my high school put me on what they called an “accelerated schedule,” in which I was taking upper level classes (even a couple of college level classes) early, and was put on a pace to graduate after my junior year. Because I was such a slacker (and I think the pressure and expectations had a lot to do with it) I ended up still needing one class to graduate after my junior year, so I took half a senior year taking that one class independent study, and taking a bunch of goof off classes like Yearbook and “Office Aid” to fill up my day.

I graduatued and Christmas and went straight into college at 17, which was not a great idea for me at the time. I ended up partying a lot, and dropping out to go and try to be a rock star. I then spent more than a decade playing in rock bands, and trying to be a guitarist/singer/songwriter before finally deciding to pack that in and go back to college once I was approaching my 30’s. I got my BA (fairly effortlessly. One of my “gifts” in both high school and college was that I had good enough recall of lectures that I never had to take notes. This always fascinated a lot of my classmates, like it was some kind of magic trick), then got married, went into AmeriCorps, started having kids, and now work as a home health aid for mentally ill people on the weekends while being a stay at home dad during the week. I never realized my potential. In retrospect, I think this is in no small part due to a cretain kind of paralysis I always got about being able to meet expectations. Subconsciously, I think I told myself it was better not to try, and to act like I was above it all and didn’t care, then to try and be proven a fraud.