the population suffer from significantly more physical disabilities than persons whose scores fall within the normal curve (Mercer, 1973b:15).

The two groups of individuals with MR are important to the issue of assessment. Physicians typically diagnose the organic cases (i.e., those with IQs below about 55-60), very early in childhood using clinical and laboratory tests, medical histories, and other evidence employed in medical diagnoses. As described in the discussion of referral in Chapter 6, more severe cases of MR are commonly enrolled in preschool programs and arrive for public school enrollment already classified as mentally retarded. The MMR or familial cases, however, have traditionally arrived for enrollment in school undiagnosed with any disability. Diagnosis as MMR occurs only after chronic and severe achievement problems are found, marked by failure to respond to normative instructional materials and methods and leading ultimately to referral and psychoeducational assessment in an effort to determine: (a) if the child has a disability and (b) a prescription for educational treatment. It is this second group of children ultimately classified as MMR over whom the role of educational assessment is most relevant.

Behavioral Dimensions Defining Mental Retardation

The dimensions of intellectual functioning and social competence (i.e., adaptive behavior) have been fundamental to most definitions of MR. The relative importance of these two dimensions, however, has varied in the different classification schemes proposed (MacMillan and Reschly, 1996). Definitions of MR adopted by the American Association on Mental Retardation (AAMR) have historically been the most influential in terms of being adopted in federal legislation and state education codes (Frankenberger and Fronzaglio, 1991). Moreover, the various AAMR definitions adopted since that of Heber (1961) reflect modest, but not insignificant, variations of that original definition, which read: “MR refers to subaverage general intellectual functioning which originates during the developmental period and is associated with impairment in adaptive behavior” (Heber, 1961:3). In subsequent revisions of the AAMR definition (Grossman, 1973, 1977, 1983; Luckasson, 1992), the importance of adaptive behavior vis-à-vis intelligence was enhanced, and the cutoff score on tests of intelligence defining “subaverage general functioning” has also varied.

Intellectual Dimension

Under Heber (1961), the criterion for subaverage general intellectual functioning was -1 SD (approximately IQ 85); however it was dropped to -2 SDs by Grossman (1973) (approximately IQ 70). Later, guidelines for employing IQ cutoff scores were adjusted, permitting identification of chil-

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