I’ve seen people say that IQ scores are supposed to be pretty stable across your lifetime, but I’m curious how much fluctuation is actually normal. Like, if someone scores 115 on one test and then 125 a few years later, is that considered a real increase or just within the margin of error?
For most modern IQ tests (like the WAIS/WISC), the 90% confidence interval is about 5 to 6 points. This means a 10-point jump, like from 115 to 125, is generally considered a statistically significant change. A difference within 5 points is usually attributed to the normal margin of error (the “noise”) and isn’t interpreted as a genuine cognitive shift.
Standard error of measurement for most IQ tests is around 3-5 points, so scores can fluctuate 5-7 points due to measurement error alone. A 10-point change (115 to 125) exceeds measurement error and could reflect real change, especially across several years with major life changes (education, brain injury, etc.). However, test-retest reliability over short periods is high (.85-.95), meaning most people stay within 5-10 points. Changes beyond 15 points are rare and warrant investigation for environmental factors, health changes, or testing validity issues.
To add, I think IQ stability varies by life stage. Scores are less stable in childhood/adolescence when the brain is still developing, but become more fixed in adulthood, making significant changes after age 25-30 less common.