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Differential Ability Scales (DAS)


Step-by-step Analysis

Step Seven: Evaluate Subtest Variability (Core and Diagnostic subtests)

  • Identify any significant subtest variation from the Mean Core T Score

A test battery such as the DAS provides a picture of an individual's cognitive strength and weaknesses. This type of evaluation is considered ipsative - within the individual. As such, when evaluating the test profile, the relative level of a subtest score, rather than the absolute level, is of great importance.

Although subtest scores are related, they differ in item content and test administration and thus these differences cause the subtest scores to vary. In statistical terms, each subtest carries with it some components of shared common variance, while most have some proportion of specific, reliable variance as well as finally components of error variance. Subtests can, and do, differ from each other. Before one can evaluate the differences between what appear to be high or low subtest scores, one must evaluate whether these apparent differences are large enough to warrant interpretation. To do so we must know if the difference is large, reliable, and significant.

Determining an individual subtest's strength or weakness requires that one examine how discrepant is each subtest from the full test mean. The DAS has provided the examiner easy access to both the child's mean for the test (found in the DAS Manual, in Tables 3 and 4) and the discrepancy requirement for each subtest (on the protocol summary page, in the DAS Manual, Table 12, as well as in the DAS Handbook, Table B.5.).

For statistical significance at the .05 level, the Preschool and School-Age core subtests require between 8 to 13 points of difference between the subtest T score and the Mean Core T score. Diagnostic subtests vary between 9 and 16 points at the .05 level.

  • Identify the frequency of any observed significant differences

Tables B.6, B.7 and B.8 in the DAS Handbook provide approximate percentages of the norm sample that obtained certain differences between the Mean Core T score and the individual subtest scores. Using these tables allows the examiner to determine whether the differences observed reach a level of unusualness. Overall, for the core subtests, differences of about 11 points on the Preschool battery and 10 points on the School-Age, would be expected to occur in only about 10 percent of the children tested.

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