Intelligence tests
Intelligence assessment is performed using tests like the Stanford-Binet test for children and the Wechsler Adult Intelligence Scale (WAIS). These two intelligence tests are widely used in the United States.
The first of them, originally created by the French psychologist Alfred Binet, was modified by Lewis Terman of Stanford University in 1916. It has afterward experienced modifications. One of the modifications introduced by Terman was the form of expressing results. A child's performance on the Binet test was expressed in terms of his 'mental age,' that is, the age that should have a child to perform as satisfactorily as the individual.
Terman considered necessary to relate the mental age to the actual age of the individual, establishing a ratio that was later called 'intelligence quotient,' or IQ, between the mental age and the chronological age. The IQ thus effectively provided an index of relative performance with respect to the individual's age group.
Nowadays the Stanford-Binet test scores are reported in terms of a norm group of the same age. They are not reported in terms of a quotient between mental age and chronological age, but rather in terms of the individual's location within the norm group, what is called a standard score. A normal distribution is assumed for the scores, with a large number of children in the middle and few with extremely high or low scores. The distribution has a mean value (or average score) of 100, and a standard deviation of 16. Scores have shown to be stable over a period of years, and this has resulted in the widespread use of this test as an intelligence measure.
The first version of the Wechsler Adult Intelligence Scale was published in 1939 by David Wechsler (1896-1981). The WAIS uses 11 sub-tests to give a verbal score, a performance score, and an overall score. Scores are also standardized with a mean value of 100.
During the 1960s and 1970s a great controversy developed concerning the relation between heredity and intelligence test scores. As part of this controversy, the American scientist Gordon Harrington demonstrated that any intelligence test is biased with relation to the psychological theory used to construct it, and to the composition of the group used to standardize the test. Therefore, any intelligence test should be standardized against a norm group chosen for reasonable comparison with the intended subjects of the test.
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