The Bivariate Normal Distribution and the IQ of Learning Disability Samples

Abstract

Several studies have indicated that the average IQ score of many learning disability samples is significantly below 100, although above the educable mental retardation range. The bivariate normal probability distribution was used to explain how selecting cases with achievement scores below a given z score cutoff (e.g., scores less than -1.0) and IQ scores above a given cutoff (e.g., IQ scores 80 or greater) can result in an average IQ score significantly below 100. Because of the relationship between achievement and IQ, as the correlation between achievement and IQ increases, the average IQ score of learning disability samples decreases. The bivariate normal probability distribution was illustrated by considering correlations of 0.0, .5, and 1.0.

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References

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