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Common intelligence tests such as the WAIS or RIAS don't differentiate well (or at all) in very low IQ ranges, making it impossible to diagnose the degree of mental retardation according to the criteria put forward in the ICD-10.

How can IQ below 40 be measured?

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  • $\begingroup$ Even to the extent that IQ is useful at all as a construct, I think it's misguided to attempt to measure IQ at the extremes. I'm not a clinician but I don't believe anyone actually uses IQ test scores to classify disability at that level of granularity, instead you should interpret those values according to their meaning relative to standard deviations from an entire population. Is your actual question of interest about IQ scores or about ICD-10 diagnostic categories? $\endgroup$ – Bryan Krause May 21 at 19:27
  • $\begingroup$ @BryanKrause - The point raised in the question is that the ICD requires IQ assessment results well below 40 for certain retardation categories $\endgroup$ – Chris Rogers May 22 at 3:51
  • $\begingroup$ @ChrisRogers But no one uses an IQ test or even attempts to do so for those clinical diagnoses, right? That's what I was asking in my comment $\endgroup$ – Bryan Krause May 22 at 5:56
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    $\begingroup$ @BryanKrause In the context of this question, I want to understand how it might be possible to measure low IQ. Because if low IQ is given as a criterion in the ICD-10, it must be possible to measure low IQ. Otherwise those diagnostic criteria wouldn't make any sense. The ICD-10 uses rather vague and subjective terms in many cases (e.g. we aren't told exactly how much "capacity for enjoyment, interest, and concentration is reduced" in depression and that assessment is left to the subjective appraisal of the diagnostician), but for IQ in mental retardation clear numbers are given. [contd.] $\endgroup$ – Randolph Eggerton May 22 at 14:10
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    $\begingroup$ [contd.] The ICD-10 even explicitly states that "[d]egrees of mental retardation are conventionally estimated by standardized intelligence tests". To me that means that the person who wrote that passage thought that all degrees of mental retardation – and therefore all ranges of IQ – may be measured by standard intelligence tests. $\endgroup$ – Randolph Eggerton May 22 at 14:12
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With great difficulty. I'm half-joking because I'm not totally sure, but the first observation is that DSM-IV also used these brackets, so the ICD-10 isn't alone.

The DSM-IV definition utilizes four degrees of severity that reflect the level of intellectual impairment: IQ levels between 50–55 to approximately 70 characterize mild mental retardation, 35—40 to 50–55 characterize moderate mental retardation, 20–25 to 35–40 characterize severe mental retardation, and IQ levels below 20–25 characterize profound mental retardation.

As far as actually testing the profound levels of retardation, it seem to me that alternative tests might be used in practice, e.g. a 2018 Texas HHS guideline says:

In some situations, an individual’s limitations may be so extensive that a full scale IQ score cannot be obtained from a standardized intelligence test. In these situations, an estimate of the individual’s IQ score, or, IQ score equivalent should be stated with clinical justification. For example, if an individual’s ability to comprehend oral instruction or visual demonstration is not adequate for a formal appraisal of general intellectual functioning, the use of an Adaptive Behavior Composite (e.g., provided by the VABS) may serve as an estimate of the individual’s intellectual functioning when accompanied by a clinical justification explained in the DID report. However, not all measures of adaptive behavior (e.g., ABAS, ICAP, and SIB) are appropriate for establishing an IQ score equivalent.

VABS is the Vineland Adaptive Behavior Scales; there's a reasonably cited paper by Bölte and Poustka (2002) correlating the ABC subscale of that with IQ < 70 (which is measured on the WISC-R/WAIS-R scale). But it also notes that the because of floor effects with WISC/WAIS we can't be certain that the substitute is really appropriate. It also cites some other papers some of which disagreed with this idea of substituting VABS for IQ.


The ICD-11 has moved away from merely using the IQ as discriminator, but has kept it. The DSM-5 was a little bolder:

DSM-5 abandoned specific IQ scores as a diagnostic criterion, although it retained the general notion of functioning two or more standard deviations below the general population.

As the Texas guideline explains further:

the DSM-5 (pages 33-36) is the first edition to classify level of severity based on adaptive functioning rather than IQ score. The DSM-5 explains that “the various levels of severity are defined on the basis of adaptive functioning, and not IQ scores, because it is adaptive functioning that determines the level of supports required.” In other words, the DSM-5 recognizes that adaptive functioning has greater practical significance because it is a better indicator of the individual’s ability to function in society.

And Texas has basically adopted the DSM-5 in this regard, classifying ability by an (approved) ABL test, of which there are several, but since that's not the main topic here... I won't detail that any further.

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  • $\begingroup$ +1 for a great answer covering the issue of IQ below 40 needed in criteria $\endgroup$ – Chris Rogers May 23 at 11:52
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    $\begingroup$ In summary I think you could conclude that although IQ numbers are used in these guidelines, they have little meaningful association with IQ in the more typical ranges of IQ tests given to the general population - would you agree? $\endgroup$ – Bryan Krause May 23 at 14:52
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    $\begingroup$ @BryanKrause: yes. $\endgroup$ – Fizz May 23 at 17:14

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