What Is IQ? A Modern Definition
IQ — short for intelligence quotient — is a normed score on a standardized cognitive test, calibrated so that the population mean is 100 and the standard deviation is 15. It is not a measure of how 'smart' a person is in any absolute sense; it is a measure of where one person's score on a particular test falls within the distribution of all people who have taken that test.
IQ — short for intelligence quotient — is a normed score on a standardized cognitive test, calibrated so that the population mean is 100 and the standard deviation is 15. It is not a measure of how 'smart' a person is in any absolute sense; it is a measure of where one person's score on a particular test falls within the distribution of all people who have taken that test.
The 'quotient' in the name is historical. Early intelligence tests, including the Binet-Simon scale of 1905, scored children by computing a 'mental age' — the age at which the average child would solve the same items the test-taker just solved — and then dividing that mental age by the child's chronological age, multiplied by 100. A 10-year-old who solved items the average 12-year-old could solve had an IQ of 120 (12 ÷ 10 × 100). The ratio approach was abandoned for adults in the 1930s in favor of deviation IQ, which compares a person's score to the distribution of scores from same-age peers — but the 'quotient' label stuck.
Modern IQ tests are deviation IQs. Your raw score (the number of items you got correct, possibly weighted by difficulty) is converted to a standard score by reference to a norming sample of test-takers in your age band. The conversion is calibrated so that 100 is the mean of that norming sample and 15 is one standard deviation. About 68% of adults score between 85 and 115; about 95% score between 70 and 130; about 99.7% score between 55 and 145. The numerical range above and below those values exists but conveys decreasing precision as you move toward the tails.
Critically, an IQ score is not a measure of any single cognitive faculty. It is a summary of performance across a battery of subtests that sample multiple distinguishable cognitive abilities — verbal comprehension, perceptual reasoning, working memory, processing speed, and so on. The full-scale IQ is essentially a weighted average of these subtest scores. A person with a full-scale IQ of 110 might have a profile that is flat across all subtests, or might have substantial peaks (say, 130 on verbal comprehension) and valleys (say, 90 on processing speed). The summary number is convenient but loses important information.
Finally, an IQ score should always be reported with a confidence interval. The standard error of measurement on a clinical IQ battery is roughly 3 points; a screener like the one on this site has an SEM closer to 7 points. A measured score of 115 on a clinical test means 'somewhere between about 109 and 121 with 95% confidence' — not 'exactly 115'. Treating IQ scores as precise point estimates is one of the most common interpretive errors made by people unfamiliar with measurement theory.