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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