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Differential Ability Scales (DAS)


Step-by-step Analysis

Step Three: Evaluate Between-Cluster Differences

  • Identify any significant differences between DAS clusters

If there are significant and unusual differences between any pair of cluster scores , the GCA must be interpreted with caution. As with almost any cognitive test that assesses multiple cognitive abilities, differences among the abilities can often lead to important interpretive hypotheses. Between-Cluster differences can occur for a number of different reasons, including learning disabilities, different interests, strengths/difficulties working under time pressures, strengths/deficits in information processing, sensory impairments, cognitive styles, or brain injury. Neither the presence nor absence of Between-Cluster differences is by itself sufficient to diagnose or rule out any disability.

The DAS Introductory and Technical Handbook Table B.1. and Table B.4. (pp. 290-292) provides the examiner with information about the difference required for statistical significance for these Between-Cluster comparisons. Rounded mean values at the .05 significance level are also found on the protocol Summary Page. For School-Age children, the mean values for each of the three comparisons is approximately 16 points, while for Preschool children the Verbal/Nonverbal mean value is 14 points.

When each of the V/NVR/Sp comparison differences is less than the critical values for significance, the GCA is most likely a reliable total. If there is no compelling reason to bypass the statistical approach to interpretation (e.g., significant scatter between the scores that make up the clusters; the effects of retesting), one may infer that the person displays fairly equal abilities whether through verbal expression of concepts, knowledge, and reasoning; through complex nonverbal inductive reasoning; or through complex visual-spatial processing.

  • Identify the frequency of any observed significant differences

As noted above, a statistically significant difference between certain cognitive abilities is often found frequently in the general population of children. If a DAS Cluster differs significantly from any other Cluster on the test, examiners must determine the base-rate frequency of the observed difference. The Between-Cluster differences shown by various percentages of the standardization sample are found in Tables B.4. and B.5. in the DAS Introductory and Technical Handbook. In general, Preschool Verbal and Nonverbal Clusters would need to differ by approximately 25 points in order to approach a level seen in only 10% of the population. For the School-Age clusters, a difference between clusters must be between 21 and 24 points to be considered unusual. It must again be noted that these tables, in the DAS Introductory and Technical Handbook, are derived from the absolute value of the difference and disregard the nature of the discrepancy (e.g., V > NVR, NVR > V). As before, to estimate the frequency of a particular specific cluster comparison in one direction, halve the size of the percentage shown in these tables.

If there are differences that are significant and unusual, interpret subtests rather than their cluster

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Step Four