On aggregate, IQ tests exhibit high reliability, although test-takers may have varying scores when taking the same test on differing occasions, and may have varying scores when taking different IQ tests at the same age.

Are IQ tests unreliable?

Intelligence tests are unreliable predictors of performance, and are inaccurate (sometimes varying by as much as 15 points from test to test.) ... The IQ score may be lower than someone who doesn't have these problems, even though they have identical reasoning and problem-solving skills.

Do IQ tests really measure intelligence?

Not necessarily. New research concludes that IQ scores are partly a measure of how motivated a child is to do well on the test. And harnessing that motivation might be as important to later success as so-called native intelligence.