Bias and error rates for premorbid IQ estimators: Comment on Veiel and Koopman (2001)

H. O. F. Veiel and R. F. Koopman (2001) advance statistical and legal theses. They correctly point out that the usual regression formula for estimating a pre-event IQ underestimates high IQs and overestimates low IQs (due to regression to the mean). They call this a conditional bias and show it can be sizeable. The author takes issue with their claim that a new estimator they propose should be used in place of the usual formulas, because it negates this statistical bias. Their argument against the usual estimator conflates statistical bias and legal bias. Their discussion in favor of their new estimator mentions, but does not derive a general formula for, a gross loss of precision entailed by use of the new estimator. The author quantifies this loss of precision and, using Veiel and Koopman's numerical example, shows that their estimator quadruples error.