Premorbid IQ estimates from a multiple aptitude test battery: regression vs. equating

Estimation of premorbid abilities remains an integral part of neuropsychological evaluations. Several methods of indirect estimation have been suggested in the literature. Many of these methods are based in prediction via linear regression. Unfortunately, linear regression has the well-reported tendency to underpredict high IQ scores and overpredict low IQ scores. This can be shown to be an unavoidable statistical artifact of linear regression. We demonstrate a procedure to estimate premorbid IQ without the regression artifact. The procedure has two steps: confirmation of construct equivalence and psychometric equating. An example using real data is presented which shows the regression to the mean problem with prediction and compares it to the results from equating.