In fact, clinical judgment has usually been employed to supplement the information on the severe and chronic achievement problems that prompted the referral in arriving at the conclusion that adaptive behavior is impaired. Garber (1988) described the situation as follows: “Definition may require that both intellectual and adaptive skill levels be ascertained....It is the low IQ scores that cause the label of mental retardation to be applied” (p. 10). Reschly (1992) observed that prior to about 1980, for school-age children and youth who accounted for the majority of detected cases of MR, “low achievement as assessed by standardized measures of achievement along with referral for academic difficulties was sufficient to constitute a deficit in adaptive behavior” (p. 33). Over the past two decades, considerable effort has been devoted to the more precise measurement of adaptive behavior in multiple contexts (Harrison and Robinson, 1995).

Disagreements over the key domains have complicated the use of adaptive behavior in decisions about MR eligibility in schools, as has uncertainty about appropriate cutoff scores to define a deficit in adaptive behavior. Adaptive behavior measures differ in underlying conceptions of adaptive behavior (e.g., the degree to which learning and achievement are important dimensions for children and youth), methods of obtaining information (e.g., third-party respondent vs. direct observation), the key contexts (e.g., home, school, neighborhood), and appropriate respondent (e.g., parent, teacher, peers, or the child himself or herself). A most vexing but enormously important issue is the selection of a cutoff score to define a deficit in adaptive behavior. The modern MR definitions refer to a deficit in adaptive behavior or deficits in adaptive skills. They include the modifier, “significantly sub-average” that is the basis for the IQ of approximately 70 to 75 on the intellectual functioning dimension. There is no modifying wording applied to adaptive behavior that provides the basis for a specific, required cutoff score for adaptive behavior. Consistent with these definitions, the deficit might be more appropriately defined through clinical judgment or a criterion such as 1 SD below the mean rather than the 2 SD criterion applied to the intellectual dimension.

The issues concerning adaptive behavior measurement are more than sterile academic debates. Research in the 1980s showed that MR was essentially eliminated if the adaptive behavior measure focused on nonschool settings, eliminated practical cognitive skills, and used parents as the sole respondents (Heflinger et al., 1987; Kazimour and Reschly, 1981). Recently developed adaptive behavior instruments generally suggest a more moderate view, in which the adaptive behavior cutoff score is somewhat flexible and decisions about the existence of deficits are based on consideration of performance over several domains (Harrison and Oakland, 2000). The evidence to date clearly supports the conclusion that the measurement of adaptive behavior is not as well developed as the measurement of general intellectual functioning.

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