dren with IQ scores up to 75, a change explained as follows: “This particularly applies in schools and similar settings if behavior is impaired and clinically determined to be due to deficits in reasoning and judgment” (Grossman, 1983:11). The most recent AAMR definition (Luckasson, 1992) has continued with “a version” of the IQ 75 upper limit: “a score of approximately 70 or 75 or below” (p. 14). This imprecision has been criticized (MacMillan et al., 1993, 1995) on the basis of the proportion of cases falling between IQ 70 and 75 (even ignoring the standard error of measurement). MacMillan and Reschly (1996) argue that while setting the upper cutoff score is arbitrary, the imprecision reflected in the Luckasson guidelines reflects a lack of awareness of psychometrics. Table 7-1 shows the consequences of these subtle shifts in IQ scores for the proportion of children eligible on the intellectual dimension defining MR alone. Very slight shifts in cutoff scores have rather dramatic consequences in terms of the percentage of the general population eligible. The proportion eligible using IQ 70 and below is only half as large as the proportion eligible using a criterion of IQ 75 and below.

The application of more or less stringent cutoff scores clearly influences the degree of overrepresentation of disadvantaged minority children in the MR category. The degree of overrepresentation would be expected to be larger when higher, rather than lower, IQ cutoff scores are employed due to the nature of the distributions of intellectual performance. Reschly and Jipson (1976) studied the effects of different IQ cutoff scores (IQ 70 and IQ 75) on the potential overrepresentation of black, Hispanic, and American Indian children. Greater overrepresentation occurred at IQ 75 and below than at IQ 70 and below. Moreover, the fact that tests of intelligence yield

TABLE 7-1 Proportion of the Population Falling Below Certain IQ Cutoffs and Falling Within Certain IQ Intervals

IQ

Normal Curve Percentage

Below 70

2.28

70 and below

2.68

Below 75

4.75

75 and below

5.48

IQ Interval

Percentage Within Interval

56-60

0.30

61-65

0.69

66-70

1.52

71-75

2.80



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