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Differential Ability Scales (DAS)


The DAS, like many other standardized cognitive ability tests, uses the Deviation IQ (M = 100, SD = 15) for the Verbal, Nonverbal, Nonverbal Reasoning, Spatial, and General Conceptual Ability composite scores and T scores (M = 50, SD = 10) for the 17 individual subtests. The calculation of the DAS scores involves a 3-step process. After each subtest is scored, raw point totals are converted to Rasch-type "ability scores." These ability scores provide a raw level of performance on the individual subtest, based on the number of correct item responses and the difficulty of the items administered. Each subtestís ability score is then converted to a T score with a range from 20 to 80. From the sums of the T scores for the subtests that create clusters, the examiner obtains the standard scores for the clusters of the test. Finally, the T scores obtained for the respective clusters are added and this sum is used to obtain the GCA score. To convert ability scores to T scores within the examinee's own age group, use Table 1 in the DAS Administration and Scoring Manual (Elliott, 1990; pp. 280-373). Age groups are 2-month intervals for children 2 years 6 months to 7 years, 11 months, and 6-month intervals for children 8 years and older.

Tables 2 and 3 (pp. 374 - 384) in the DAS Administration and Scoring Manual are used to obtain Cluster and GCA scores based on the battery-appropriate standard subtests. If the examiner takes advantage of the opportunity to test a child older than age 5 years 11 months with the Preschool Battery or a child younger than age 6 years with the School-Age battery, it is essential to use the Cluster and GCA tables that correspond to the battery actually given, not the tables for the child's age. Diagnostic subtests are never used in the calculation of GCAs nor used as replacements for core battery subtests.

Test-Age Equivalents

Table 11 (pp. 408-9) in the DAS Administration and Scoring Manual provides, for the all Cognitive and Achievement subtests, age-equivalent scores that reflect the age at which the examinee's ability score is the median score. No age-equivalent scores are available for the clusters or the General Conceptual Ability Scores. These scores are provided to assist the examiner in interpreting scores on subtests that have been administered to children at ages for which there are no norms. Extreme caution is advised whenever using or interpreting these scores.