Each of these causes may have different effects on the intelligence and ability of individuals with retardation, and experts have made various attempts to classify the degree of these effects. The most common attempts assess only intellectual functioning, despite the fact that mental retardation involves both intellectual and adaptive skill levels. Since the early 20th century, schools in the United States have used intelligence testing to determine appropriate educational programs for children with mental retardation (see Psychological Testing: Intelligence Testing).
A commonly used testing system has five classifications of mental retardation based on the intelligence quotient (IQ) of a given child, with an IQ of 100 considered average: (1) mild retardation, with IQ scores of 50 to 70; (2) moderate, with scores of 35 to 50; (3) severe, with scores of 20 to 40; (4) profound, with scores below 20; and (5) unspecified severity, where retardation is suspected but the person cannot be tested by standard intelligence tests.
The results of these tests determine the extent of special educational services required by the student. In general, the lower the score on an intelligence test, the greater the degree of specialized education the student receives. In addition, students with lower scores will generally require special education services in restricted environments, away from the regular classroom.