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Discrepancies Between Different Ability Tests - WISC and CogAT tests

By Inderbir Kaur Sandhu, Ph.D

Q: I'm wondering about variations in scores between the CogAT and WISC tests. We've seen considerable variation, which is confusing!

As a kindergartener, we had our son tested by the school system as we were trying to figure out what school would serve him best. He tested as talented and gifted in math, but scored in the 89th percentile on CoGAT test.

We chose schools poorly, unfortunately, and he started having significant trouble in 2nd grade. We took him to an educational consultant. His WISC scores from that time were:

  • Verbal Comprehension: 155, 99.9th percentile

  • Perceptual Reasoning: 149, 99.9th percentile

  • Working Memory: 104, 61st percentile

  • Processing Speed: 112, 79th percentile

  • Full Scale IQ: 143, 99.8th percentile

We weren't sure if his tester was reliable, since these scores seemed so high. In third grade his school administered the CoGAT again and he scored in the 94th percentile.

We went to a different educational/psychological consultant when he was in 5th grade to confirm a ADHD diagnosis. She gave him the WISC again. Here are his scores:

  • Verbal Comprehension: 138, 99th percentile

  • Perceptual Reasoning: 147, 99th percentile

  • Working Memory: 99, 47th percentile

  • Processing Speed: 118, 88th percentile

  • General Ability Index: 153, 99.9th percentile

Why would we see such a big difference between scores? This discrepancy has caught my attention again because his 6-year-old sister was just tested by the school and she scored in the 91st percentile on the CoGAT. I'm wondering if we should have her tested with the WISC. The cut off for talented and gifted services in our district is a 97th percentile score.

Thank you!

A: Just a short note on the CogAT. Briefly, it is a measure of a student's potential to succeed in school-related tasks. It should not be confused as a measure for intelligence or IQ per se even though it is considered as an intelligence test. Rather, it measures the reasoning skills that have developed even though these general cognitive skills are probably not explicitly taught. They may not be specific to any content area, but these are skills used in all areas of a student's academic experiences. Apart from general cognitive skills, the CogAT also measures general school skills (e.g., ability to listen, follow directions, and focus attention). It can beneficial when looking for the student's ability to be successful in school and to look for dominant learning strengths.

Teachers commonly use it when they have concerns over a student's classroom performance or lack of progress. The scores provide a general sense of a student's abilities and learning styles, which may lead to further analysis. Bear in mind that the CogAT is an aptitude test and measures preparedness for academic work, not the result of instruction in school. Even though there is a correlation between innate ability and one's performance, the CogAT is not an IQ test per se.

CogAT is commonly used as screening measures, to see if the child should move to a full gifted assessment (individual intelligence test). These tests are commonly administered by teachers and generally normed on populations of all children, with relatively few gifted children among the mix. It has been noted that gifted children tend to over-think the questions when taking group tests and this may cause wrong selections of options. Furthermore, as there is no individual tester to clarify some unique (but acceptable) responses, there is a high possibility for gifted children to often score lower than expected on group intelligence tests.

Tests measure different abilities and have different purposes. Scores on ability tests may differ due to a few reasons such as the fact that different tests measures aspects of the same ability; tests are normed on different group of students and the possibility of errors of measurement to occur. Therefore, one should not expect that children must have similar scores on different ability tests. Main reason for the difference of scores between two ability tests administered to the same child is due to the somewhat different measures of skills of the same ability construct. The CogAT measures abstract reasoning abilities in the three symbol systems most required for learning in school such as verbal, quantitative, and figural/spatial. Therefore, it has been noted that CogAT generally predicts school success better than individually administered ability tests.

I certainly believe that your son is gifted and would require differentiation in learning. His high scores on VCI and PRI and lower scores on WMI and PSI is typical for a number of gifted children. Hence the GAI score is appropriate. For your daughter, perhaps it would be good to get a standardised IQ test administered. Bear in mind that CogAT composite score is considered to be most meaningful when the Verbal, Quantitative, and Nonverbal Battery scores are all at about the same level (check the breakdown of her scores). If it is not even and displays unusual scatter, the composite score (and percentile) should not be regarded as very dependable.

I hope your children will get an ability based educational match. Hope that helps and wishing you all the best for both your children.


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