Accuracy of regression equation prediction across the range of estimated premorbid IQ

Linear regression is often used to predict psychological criterion variables such as premorbid IQ. The prevailing method of evaluating the accuracy of prediction indicates poor accuracy for both high and low criterion values. These results have led to the conclusion that the equations are not applicable for predicting scores beyond about one standard deviation from the mean. The apparent inaccuracy at the extremes, however, is an artifact of inappropriate analysis. An alternative analysis method is described and used to re-analyze two sets of data. Empirical results show that both high and low WAIS-R IQ scores, predicted using versions of the NART, agree with actual measured IQ scores as accurately as do predicted scores near the mean. In addition, confidence intervals were only 5% larger for more extreme predicted values than for values near the mean, which would be of minor clinical consequence. The practical constraint on prediction of extreme values arises not from the regression technique, but from the limited range of the predictor variable(s). The two reviewed IQ prediction equations would, however, have adequate range for a high percentage of individuals.