The first English-language IQ test, the Terman-Binet, was adapted from an instrument used to measure potential to achieve developed by Binet in France. Terman translated the test and employed it as a means to measure intellectual capacity based on oral language, vocabulary, numerical reasoning, memory, motor speed and analysis skills.
The mean score on the currently available IQ tests is 100, with a standard deviation of 15 (WAIS/WISC-IV) or 16 (Stanford-Binet). Sub-average intelligence is generally considered to be present when an individual scores two standard deviatons below the test mean. Factors other than cognitive ability (depression, anxiety, etc.) can contribute to low IQ scores; it is important for the evaluator to rule them out prior to concluding that measured IQ is “significantly below average”.
The following ranges, based on Standard Scores of intelligence tests, reflect the categories of the American Association of Mental Retardation, the Diagnostic and Statistical Manual of Mental Disorders-IV-TR, and the International Classification of Diseases-10:
Profound mental retardation Below 20
Severe mental retardation 20–34
Moderate mental retardation 35–49
Mild mental retardation 50–69
Borderline intellectual functioning 70–84
Since the diagnosis is not based only on IQ scores, but must also take into consideration a person’s adaptive functioning, the diagnosis is not made rigidly. It encompasses intellectual scores, adaptive functioning scores from an adaptive behavior rating scale based on descriptions of known abilities provided by someone familiar with the person, and also the observations of the assessment examiner who is able to find out directly from the person what he or she can understand, communicate, and the like.