Wallin: A mentally retarded individual is one who "on standardized tests fails to attain an IQ or MA of a particular level" (1949, p. 13).

Ingram: "The term 'slow learning' is used by many as a designation for any child who cannot meet average grade academic standards year by year. This group comprises approximately 18 to 20 percent of the school population—those who measure approximately 50 to 89 IQ on individual standardized intelligence scales. Within this classification the terms 'borderline' or 'dull normal' are generally applied by the psychologist to those who measure approximately 75 to 89 IQ. This is the larger group, comprising 16 to 18 percent of the school population. The terms 'mentally retarded' or 'mentally handicapped' are applied to those who measure approximately 50 to 75 IQ, the lowest 2 percent of the school population in learning ability"

Spitz: "Mental retardation: a condition of retarded mental development as determined by an IQ below 70 on a standardized individual intelligence test—a condition which, to the best of our knowledge, has existed from before the age of 3" (1963, p. 12).

Zigler: "The essential defining feature of mental retardation is intelligence lower than that displayed by the modal number of an appropriate reference group" (1973, p. 231).

Statistical model: "Mental retardation is defined as a deviant range on a continuum of intellectual ability, normal status then being defined as the average of middle range."

Source: Neisworth, J.T. & Smith, R. M. (1978.) Figure 4-7, Chronological Listing of Defenitions. In Retardation: Issues, Assessments, and Intervention (pp. 66-69). New York: McGraw—Hill, Inc.