The bias in regression-based indices of premorbid IQ

Formulas for premorbid intelligence estimates are typically derived by linear regression and are therefore biased in individual cases because of regression to the mean. It is shown that it is inappropriate to compare such IQ estimates with current IQ scores to determine whether a decline from premorbid levels has occurred. This widespread practice grossly overestimates the probability of an IQ decline in the below-average range and grossly underestimates it in the above-average range, with serious implications for clinical practice. The authors present a formula for computing unbiased estimates of IQ decline as well as a test of the null hypothesis of no decline. Corresponding tables for several combinations of test indices and estimation methods are included for practical reference.