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The criterion score used in the statistical analysis is an index of over- or under-achievement.

It is the discrepancy between the actual attained achievement test score and the score that would be predicted by the I.Q..

For example, on the basis of the regression equation, a child with an I.Q. of 120 in this sample would be expected to earn an achievement test score of 4.8 ( grade equivalent ).

If a child with an I.Q. of 120 scored 5.5 in achievement, his discrepancy score would be, representing of one year of over-achievement.

A child with an I.Q. of 98 would be expected to earn an achievement test score of 3.5.

If such a child scored 3.0, his discrepancy score would be -.5, representing of one year of under-achievement.

In this manner, the factors measured by the intelligence test were controlled, allowing discovered differences in achievement to be interpreted as resulting from other variables.