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**Determining Predicted
Achievement**
When determining predicted achievement, the Table
provided take into account both the reliability of each measure used and the
correlation between the ability and achievement.
The correlation between two tests is important when defining
a severe discrepancy. Tests of ability and achievement do not correlate
perfectly (measure the same thing). Just because someone has a low ability
score, we should not assume that they would also, automatically, have an equally
low score on some other, different measure. No one would assume that just
because a person had a very high IQ that the person would obviously be able to
excel at gymnastics, or be able to hold his or her breath longer than
"normal." Just because someone is good or bad at one thing, it does
not guarantee that the person will also be good or bad at other things. An
ability (IQ) test does not measure the same thing as an achievement test. Common
statistical methodology, utilizing concurrent validity coefficients,
demonstrates that a child obtaining an extreme IQ score (e.g., 130) would be
expected to obtain a mean achievement level of only about 120. Thus the expected
achievement of the child with an IQ of 130 is not 130 at all, but more likely to
be about 120. For child with a low IQ score, the reverse is true: the expected
achievement of a child with an IQ of 70 would be approximately an achievement
score of 80. If practitioners do not adjust (regress) the scores they obtain in
determining the "expected" achievement score, they will produce
over-identification of high IQ children and under-identification of low IQ
children. Without accounting for the correlation between two tests, one would
never be able to know what the "expected" score should be.
The best way to determine the correlation between two
measures is to look the correlation up in the manual of the test used.
Unfortunately, not all manuals offer that information; the information is often
based on absurdly small samples; and not all tests have been compared to each
other. One way around this shortcoming is to estimate the correlation between
the two tests. If we know the reliability of the ability test and the
reliability of the achievement test, the correlation between the two tests can
be estimated.
The equation for estimating the correlation between two tests
is:
**The values rxx and ryy are the internal consistency
reliability coefficients for the aptitude and achievement tests used.
**
**Using the Tables to Determine the Expected Achievement
The determination of expected achievement involves 2 basic
steps:
1. Determine the estimated correlation between tests
2. Determine the expected achievement score
Go to Tables for Expected Achievement
Go to Determination of Severe
Discrepancy |