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Error in IQ Test Scores

By Inderbir Kaur Sandhu, Ph.D

Q: Our daughter took the WPPSI at age 4 years and 6 mos. She is now 9 years 5 months. What is the % of error in the test scores? Does an individual IQ score change? Why do school districts not recognize IQ as an appropriate measure of giftedness? Any information would be helpful.

A: Usually, the percentage of error in standardized tests is taken into account in the percentile band. The error of measurement here refers to the amount by which the score that is actually observed differs from a hypothetical true score. A standard error of measurement is the amount an observed score that is expected to fluctuate around the true score. For example, the obtained score will not differ by more than plus or minus one standard error from the true score about 68% of the time. For about 95% of the time, the obtained score will differ by less than plus or minus two standard errors from the true score.

A score without error is a true score, which is entirely free of error; a hypothetical value that can never be obtained by testing, since a test score always involves some measurement error. A true score may be thought of as the average of an infinite number of measurements from the same or exactly equivalent tests, assuming no practice effect or change in the person taking the test during the testing. The standard deviation of this infinite number of scores is known as the standard error of measurement.

Gifted programs in schools usually have their own measures in determining students who will benefit most from such programs. Achievement tests are perhaps more reliable in screening students for such programs. Some students may have very high IQ, but the gifted program may not suit them very well. My feel is that school districts need to practice some kind of standardization in screening children for gifted programs; hence only results/scores from certain tests taken in certain centers may be accepted. In your daughter's case, the test has been taken five years ago; hence there may be a slight change in IQ on a revised version. It is best to take another standardized IQ test if you need to place her in certain programs. You should also speak to the school authorities as they may be the best source for information in this case.


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