IQ classification
| Intellectual disability | |
|---|---|
| Classification and external resources | |
| Specialty | psychiatry |
| ICD-10 | F70-F79 |
| ICD-9-CM | 317-319 |
| DiseasesDB | 4509 |
| MedlinePlus | 001523 |
| eMedicine | med/3095 neuro/605 |
| MeSH | D008607 |
IQ classification is the practice by IQ test publishers of labeling IQ score ranges with category names such as "superior" or "average". There are several publishers of tests of cognitive abilities. No two publishers use exactly the same classification labels, which have changed from time to time since the beginning of intelligence testing in the early twentieth century.
IQ scores have been derived by two different methods since the advent of cognitive ability tests. The first method historically was the "ratio IQ", based on estimating a "mental age" of the test-taker (rounded to a specified number of years and months), which was then divided by the test-taker's "chronological age" (rounded to a specified number of years and months). For example, a mental age score of thirteen years and zero months for a test-taker with the chronological age ten years and zero months results in a quotient of 1.3 after doing the division. The division result was then multiplied by 100 so that scores could be reported without decimal points. Thus the score in the example would be reported as IQ 130 .
The current scoring method for all IQ tests is the "deviation IQ". In this method, an IQ score of 100 means that the test-taker's performance on the test is at the median level of performance in the sample of test-takers of about the same age used to norm the test. An IQ score of 115 means performance one standard deviation above the median, a score of 85 performance one standard deviation below the median, and so on.Lewis Terman and other early developers of IQ tests noticed that most child IQ scores come out to approximately the same number by either procedure. Deviation IQs are now used for standard scoring of all IQ tests in large part because they allow a consistent definition of IQ for both children and adults. By the current "deviation IQ" definition of IQ test standard scores, about two-thirds of all test-takers obtain scores from 85 to 115, and about 5 percent of the population scores above 125.
Even before IQ tests were invented, there were attempts to classify people into intelligence categories by observing their behavior in daily life. Those other forms of behavioral observation are still important for validating classifications based primarily on IQ test scores. Both intelligence classification by observation of behavior outside the testing room and classification by IQ testing depend on the definition of "intelligence" used in a particular case and on the reliability and error of estimation in the classification procedure.
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