Average IQ – Uninformative and often terribly misleading

On the Wechsler Intelligence Scales for children and adults, an average IQ score lies between 90 and 110. Well, that’s not exactly right. Some assign the average label to IQ scores between 85 and 115.

But it is not the numbers that matter, not at all. It’s HOW they were calculated. Consider: A 19 year old coed, Alice,  obtains an IQ scores of 102. That’s “average”. But that score of 102 represents a composite of at least 10 subtest scores. These subtests each purportedly measures a different intellectual ability. So, for instance, a subtest called “Information” measures a construct referenced as “long term memory” or verbal memory or declarative memory. I’ll call them all “Verbal Memory”. The items on this subtest are designed to draw on this verbal memory. The examinee is asked questions like, “Who was Thomas Jefferson?” or “How many feet are in a yard?” And the questions are arranged in order of increasing difficulty. The more you get right, the higher your raw score. But the raw score is meaningless because it doesn’t define the relative magnitude of an examinee’s verbal memory vis-a-vis a same-age population. To do this, the raw score is converted to a “Scale Score”. And these express the magnitude of the examinee’s verbal memory relative to age peers.

Now a scale score between 8 and 12 is considered average; it falls between the 25th and 75th percentiles.

Let’s say Alice gets a scale score of 10 on the Information subtest. What can we say?  What have we learned? How does this differentiate Alice from others? Or help define her specfic  educational needs?  Very, very little to not at all are the most appropriate answers to these questions.

Bad enough. But worse is the fact that scale scores from 10 such subtests are collapsed into one number called Full Scale IQ. But those scale scores can range from 1 to 19! So Alice might have the following scale scores on the ten subtests: 10, 9, 5, 14, 11, 15, 8, 9, 10 and 7. Add them up, convert them to a full scale score of 102 and Alice turns out to be “average”!!  And recalling that Alice’s patterns of responses to different subtest tasks can vary widely ( a subject I will discuss at length in future blogs), these scores themselves call into question the usefulness of each  of the ten subtests

In short, an “average” IQ can  arise from widely varying subtest scale scores that themselves can arise from widely varying response styles and patterns as different subtests are administered . In too many cases, then, an “average” IQ not only ignores marked intra-individual differences but it can, in doing so, fail atotgether to identify real strengths and disabling deficits.

In later blogs, I will explain how such remarkable variations arise within a single “Average” human being.


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