IQ tests are nearly twice as accurate as measurement of blood pressure, blood cholesterol and diagnosis based on chest X-rays.

Unlike other misconceptions about intelligence, there is some truth to the claim that IQ tests are unreliable, in the sense that IQ tests are not perfectly reliable.  IQ tests have some measurement errors, which is why psychometricians perform factor analysis to eliminate such random errors in measurement.  So it is true that IQ tests are not perfectly reliable, but then no scientific measurements ever are.

If the same individuals take different IQ tests on different days, or even on the same day, their scores will be slightly different from test to test (but only slightly).  So IQ tests do not give a perfect measurement of someone’s intelligence.  But then, if you step on the bathroom scale, get the reading, step off, and step on it again, it will give you slightly different readings as well.  The same is true if you measure your height, your shoe size, and your vision.  No measurement of any human quality is ever perfectly reliable.

So the measurement of intelligence is no different from measurement of any other human trait.  But nobody ever claims that, because the measurement of weight is never perfectly reliable, there is no such thing as weight and weight is a culturally constructed concept.  But that’s exactly what people who are unfamiliar with the latest psychometric research think about intelligence.  Intelligence is no less real than height or weight, and its measurement is just as reliable (or unreliable).

In fact, Arthur R. Jensen, who had long been the greatest living intelligence researcher until his recent passing, claims that IQ tests have higher reliability than the measurement of height and weight in a doctor’s office.  He says that the reliability of IQ tests is between .90 and .99 (meaning that random measurement error is between 1% and 10%), whereas the measurement of blood pressure, blood cholesterol and diagnosis based on chest X-rays typically has reliability of around .50.

Reliability is the correlation coefficient between repeated measurements.  If the measurement instrument is unbiased (as IQ tests are as a measure of general intelligence and the sphygmomanometer is as a measure of blood pressure), then the reliability translates into the correlation coefficient between the true values and the measured values.  The reliability of .50, for example, like the reliability of the sphygmomanometer as a measure of blood pressure, means that the correlation between individuals’ true blood pressure and the readings on the sphygmomanometer is only .50.  In contrast, the reliability of .90-.99, for example, the reliability of IQ tests as a measure of general intelligence, means that the correlation between individuals’ general intelligence and their IQ test scores is .90-.99.  So the measurement of intelligence is nearly twice as accurate as the measurement of blood pressure, yet nobody ever claims that blood pressure is not real or that it is a culturally constructed concept.

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Read more about the surprising facts about intelligence, what it is (and what it is not), how it affects you in virtually every sphere of life, and how more intelligent people are different from less intelligent people, in The Intelligence Paradox:  Why the Intelligent Choice Isn’t Always the Smart One.