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Differential Ability Scales (DAS)


This section addresses an example of how to interpret the DAS following a step-by-step interpretive method.

Rigid adherence to a g theory of intelligence would focus attention exclusively on the GCA score of the DAS. The most extreme application of the various separate-factor theories of intelligence (s) might lead interpreters of the DAS to examine the 6 subtests, or even items within those subtests, for evidence of specific abilities. One difficulty with such a fine analysis is that reducing the number of items diminishes reliability. The GCA is more reliable than the verbal, nonverbal reasoning, or spatial cluster scores. Almost any group of subtests is more reliable than a single subtest.

An intermediate approach may be the wisest. Much as Elliott (1990) and others recommend, it makes sense to attack a DAS profile hierarchically, beginning with the most reliable groupings of subtests and working through successively less reliable, smaller groupings before reluctantly finishing, as a last resort, with the least reliable data: individual subtests. If the evaluator adheres to a g theory, that analysis would be considered an investigation of deviations from the studentís overall intellectual ability. An s orientation would consider the process one of separating the studentís levels of intellectual abilities in various factors or "intelligences."

If each of the three clusters is coherent Ė tightly clustered within itself and separate from other factors Ė the three factors would be the most reasonable level of interpretation of that studentís DAS. That is, if the subtest scores within the Verbal, Nonverbal Reasoning, and Spatial Clusters are each tightly grouped, and the clusters scores differ significantly and uncommonly from one another, then it makes sense to consider the verbal, nonverbal, and spatial domains separately: the student appears to demonstrate significantly different levels of ability when dealing with verbal, nonverbal, and spatial tasks.

Sometimes, however, we need to look further. One or more of the three clusters may not be coherent and may require additional subdivision. If there is notable scatter within one or more of the Clusters, the next level of interpretation might be the narrow ability interpretation, which separate cognitive processes into more specific abilities. This is not an excuse to leap ahead to analysis of individual subtests. It is, instead, a signal to cautiously consider other groupings of subtest scores. 

What this approach and many others share in common is the idea that, especially for learning disabled students, it is not reasonable to accept the "Full Scale" GCA (BCA, IQ, GCI, MPC, SAS, etc.) as the only measure of a student's intellectual ability. Instead, interpretation must take into account strengths and weaknesses, and must recognize the fact that a specific learning disability can affect intelligence test scores as well as other measures of ability and achievement. The analysis is preferable to abandoning the intellectual assessment altogether (e.g., Siegel, 1989). The following illustration is intended to demonstrate this argument. It is absolutely not intended to suggest that this is the only way to carry out the necessary analysis.

The interpretive steps outlined and recommended below (Table XXX-23) involve a complete and thorough analysis of all the DAS data and results. The interpreter must evaluate the DAS results utilizing a practical as well as statistical approach. After the interpreter has completed all the steps necessary for making logical decisions, hypotheses can be generated from the results.

Most interpretive schemes (e.g., Kaufman, 1994; Sattler, 1992) begin with the most general aspects (global scores) and progress to more detailed aspects of the individual's performance (factors or clusters, subtest variability, qualitative responses). These procedures allow for both a quantitative and qualitative interpretation of the test, which may lead to an understanding of how the person obtained the results and performed the tasks presented by the test (Kaplan, 1988). As one moves through the successive steps presented here, readers are encouraged to consult the cluster and subtest information included earlier in the chapter. That section may provide relevant information about the DAS subtests as well as possible interpretive hypotheses and various strategies for expanding understanding of the person's underlying processes.

The approach to test interpretation that is offered here is based upon a statistical and actuarial approach that leads into hypothesis generation. Without this statistical approach, any interpretation would be less valid and reliable, and would be more likely to be inaccurate. By developing interpretive hypotheses that are based upon a statistical and actuarial analysis of the data and then coupling these findings with clinical observation, evaluators are able to make statements about a child's abilities relative to others of the same age as well as make statements based on the child's own performance. As hypotheses are generated., they are checked by testing them against the child's test performance and behaviors found on this and other tests as well as other sources of information, such as interviews, historical data, and classroom observations.

When interpreting any test, it is important to remember that the most valid interpretations are based upon the most reliable aspects of the test. Interpretation should focus on the most general areas before moving to the less general areas. In the case of the DAS, the most general and most reliable areas are the General Conceptual Ability (GCA), and then the Verbal, Nonverbal Reasoning, and the Spatial cluster scores. Below these measures are the Shared Ability factors, and finally the individual subtests.


Successive steps in the interpretation of the DAS

Step One: Evaluate the GCA

Step Two: Evaluate GCA-Cluster Differences

  • Identify any significant differences between the DAS GCA and each Cluster (Verbal, Nonverbal Reasoning, and Spatial)
  • Identify the frequency of any observed significant differences

If there are differences that are significant and unusual, interpret Clusters rather than the GCA

Step Three: Evaluate Between-Cluster Differences

  • Identify any significant differences between DAS Clusters (Verbal vs. Nonverbal Reasoning vs. Spatial)
  • Identify the base-rate frequency of any observed significant differences

If there are differences that are significant and unusual, interpret Clusters rather than the GCA

Step Four: Evaluate Within-Cluster Differences

  • Identify any significant Within-Cluster differences
  • Identify the base-rate frequency of any observed significant differences

If there are differences that are significant and unusual, interpret narrow abilities rather than the Cluster

Step Five: Narrow Ability Hypotheses

  • Identify the narrow abilities assessed and any relevant differences between them

Step Six: Evaluate Shared Ability Hypothesis

  • Identify any relevant shared ability groupings

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

  • Identify any significant subtest differences from the Mean Core T Score
  • Identify the base-rate frequency of any observed significant differences

Step Eight: Evaluate Qualitative Responses

The DAS provides the examiner many opportunities to observe meaningful, clinical behaviors. Examiners should generate and test hypotheses based not only upon the resulting scores but also on these relevant behaviors.

Since each successive step of a DAS interpretation requires examiners to judge the adequacy of certain scores, an analysis (without any hypotheses generated) should first be completed. The DAS Summary Page contains information to aid examiners with this task (critical significance values, Mean Core T score, etc). Examiners may also find the DAS Analysis Sheet (Exhibit-4) useful when beginning any interpretation of a DAS protocol. A completed DAS Analysis Sheet summarizes the statistical results that can then be used in each of the successive steps.



Step-by-step analysis