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Percentile Difference on the WISC-IV

By Inderbir Kaur Sandhu, Ph.D


Q: On the WISC-IV Canadian norms, my son scored 99.9 percentile on General Ability. On Verbal Comp. he scored 99.9 percentile. On Perceptual Reasoning he scored 99 percentile.

Can you tell me what this means in terms of IQ? What's the difference between 99 and 99.9 - and is it important. In our area, the psychometric results don't attach an IQ number to these percentiles, but at each teacher interview, I keep hearing how, even in a class of Gifted Children, he is far ahead yet he struggles with Math.....

From, Toronto - Ontario Canada

A: From the scores, it does appear that your son is in the highly gifted range with such high scores. Therefore, he would surely need special attention. I believe that the tester used General Ability Index, GAI instead of Full Scale IQ (please refer to another reader's query on GAI) to remove the processing speed and working memory which may be affected if a child struggles with Math. GAI is a composite score that is based on Verbal Comprehension and Perceptual Reasoning subtests, and does not include the Working Memory or Processing Speed subtests which is included in the (FSIQ).

An IQ score is not attached to the percentile but the detailed interpretation should have the full scale IQ which gives a general idea of the child's IQ based on the test taken. In your case, there should be a score for GAI. You may want to find out from the tester.

The percentile is based on chronological age. It indicates a child's standing as compared to other children of same age group. According to the WISC technical reports, percentile ranks reflect points on a scale at or below which a given percentage of scores lie, based on the standardization sample. In this case, say, a score of 99 percentile indicates that your son's score is better than 99 per cent of the sample that was tested (this makes the norms).

This document may help you understand the WISC Canadian Norms in detail.

You should discuss the interpretation of his detailed results with an educational psychologist who may be able to test if he has any other learning difficulties that affects his Math. Once any deficiency is found, intervention would be necessary to lessen the struggle and improve learning. Here's wishing you all the best in bringing up this very bright child!

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Essentials of WISC-IV Assessment
Dawn P. Flanagan Ph.D, Alan S. Kaufman Ph.D

The WISC-IV is the top intelligence assessment instrument for children in the US, providing essential information into a child's cognitive functioning. This book applies a new, expanded theory-based approach to interpreting the latest edition of the WISCŪ and provides beginning and seasoned clinicians with comprehensive step-by-step guidelines to administering, scoring, and interpreting this latest revision of the Wechsler Intelligence Scale for Children.

This book provides students and practitioners with an unparalleled resource for learning and application, including expert assessment of the test's relative strengths and weaknesses, valuable advice on its clinical applications, and illuminating case reports.

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