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IQ and Standardized Tests

By Inderbir Kaur Sandhu, Ph.D


Q: Is there anywhere on the results of the Terra Nova tests whereby I can see what my child's IQ is?

Our son has been tested in the past and has an IQ on previous tests of 147. He is in gifted programs etc. When I asked for the IQ on the Terra Nova, I was called and told a number which is not near where he was previously, yet his test results show high levels into the 90 on Terra Nova test. Please advice.

A: To be placed in the gifted program, a child would, on the average, have a composite score of at least 96th percentile on a standardized achievement test, which is usually the Terra Nova that is widely used. Another criterion would be a cut-off score from an intelligence test. For example, for the WISC, 4th Ed., a minimum full scale score of 128 considers a child eligible for the gifted program.

As you can see here, one is by means of an achievement test and another is an intelligence test. Terra Nova is in fact a comprehensive test of basic skills while most intelligence tests measure certain thinking skills that are mostly school related, but may not be school specific. IQ tests are generally thought of as a kind of "ability" test in contrast to achievement tests. IQ is a score derived from a set of standardized tests developed to measure a person's cognitive abilities ("intelligence") in relation to their age group. Hence, I am not sure what number was given to you as a conversion from an achievement test to an intelligence quotient. I really think that the two types of tests should not be confused in terms of their results.

However, there has been some method employed for the conversion which I really do not believe is accurate. Here goes:

Apparently, you can convert a scale score which is usually derived from an achievement test to an IQ score, though this is based on assumption of a normal curve. A distribution of scaled scores has an average of 0.0 and a standard deviation of 1.0. This is pretty standardized. If you multiply all the scaled scores by 100 and add 15, you should get the common IQ scale - which is an average of 100 and a standard deviation of 15.

If you multiply them by 100 and add 500 you should get the scale of the SAT - a mean of 500 and a standard deviation of 100. Scaled scores also allows normative comparison of scores across different scales, For example, an IQ score of 115 is considered equal to an SAT verbal score of 600 because both are one standard deviation above the average, which are 100 and 500, respectively. A person with an IQ score of 115 and an SAT verbal score of 600 has scored at the 84th percentile on both tests.

However, since they are two different tests measuring different skills, it would be better not to convert the scores. In addition, different IQ tests may show different ceilings or cut-off points for above-average IQ. Therefore, it is best to go for standardized intelligence tests if you want an IQ score. You should consult the administrator of his Terra Nova test to explain the discrepancy, which I believe may be due to the fact that the Terra Nova is an achievement test to test basic school-learnt skills, and not meant to test IQ scores.


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