How big is the SEM on a clinical IQ test?

Clinical full-scale IQ scores carry a standard error of a few points. Manuals turn that into a confidence interval, often around plus or minus 5 points at 95%.

Is SEM the same as standard deviation?

No. The standard deviation describes how spread out a whole group is (15 points on the IQ scale). The SEM describes the uncertainty in one person's score.

Glossary

Standard error of measurement

Updated October 21, 2026

Definition

The standard error of measurement, or SEM, estimates how much a person's observed score would vary around their true score if they could take the test again and again without practice effects. A smaller SEM means a more precise score. It is used to build confidence intervals.

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Full explanation: How accurate is an IQ score?.

Why it matters for IQ scores

The SEM is the reason a careful IQ report gives a range. Clinical full-scale scores carry a standard error of a few points, and manuals report a confidence interval, often around plus or minus 5 points at 95% (Wechsler, 2008). A 95% interval is roughly the score plus or minus about two SEMs.

An example

Suppose a test has an SEM of 2.5 points and you score 104, a 95% interval runs roughly from 99 to 109. Shorter tests tend to have larger SEMs, because each item carries more weight. In item response scoring the same idea appears as the standard error around an ability estimate, and it is usually larger at the extremes where fewer items match the person's level.

Common confusions

The SEM is not a sign that a test is bad. Every measurement has one; good reports simply show it. It is also different from the standard deviation of the scale. Related terms: confidence interval, reliability and theta. For how IQ Octopus reports its range, see how the test works or how to read an IQ score.

Try one · Abstract Reasoning

Which tile completes the grid?

Tap the option you think is right, or show the answer.

Show the answer

The missing tile has two solid circles (option 5).

How to solve it

Start with the first row and name what changes from tile to tile.
Every row and every column contains each shape (square, circle and cross) exactly once.
Every row and every column contains each number of shapes (two, one and three) exactly once.
The fill (solid) stays the same in every tile.
Check the rules on the second row, then apply them to the third.
So the missing tile has two solid circles.

That was 1 puzzle. The full test has 25.