The Binet‐Simon intelligence scale, developed by French psychologists Alfred Binet and Theodore Simon, was administered to children to evaluate their performance ( mental age) at a given chronological age. The mental age/chronological age measure, called a mental quotient, was used to evaluate a child's learning potential.
Lewis Terman of Stanford University revised the Binet scale in 1916. The revised scale, called the Stanford‐Binet intelligence scale, although it retained the concept of mental and chronological ages, introduced the concept of the intelligence quotient (IQ) arrived at by the following widely used formula, which allows comparison between children of different ages.
The 1986 revision of the test, the latest of several, varies the calculation so that the test is useful for adults as well as for children. An individual's score for correct answers is compared to a table of scores of test takers of the same age (with the average score always scaled to 100). Scores between 90 and 110 are labeled as “normal,” above 130 as “superior,” and below 70 as mentally deficient, or “retarded.” The distribution of IQ scores approximates a normal (bell‐shaped) curve (Figure ).
The Normal IQ Distribution
David Wechsler developed the Wechsler Adult Intelligence Scale (WAIS) in 1939, revised as the WAIS‐R. Wechsler also developed the Wechsler Intelligence Scale for Children (WISC), revised as the WISC‐R. The revised forms of these scales are still widely used. They contains two sub‐scales, verbal and performance, which provide a verbal IQ and a performance IQ; the subscales are combined for the total IQ. Test score combinations may reveal other strengths and weaknesses to a skilled examiner.