Will the real discrepant learning disability please stand up?

Willson and Reynolds (in this issue) challenged my thesis that the regression-based discrepancy method (RDM) is not a valid tool to detect aptitude-achievement discrepancies. In this response, I show that the statistical and theoretical counterarguments of Willson and Reynolds are based on a misreading of the statistical models presented. Furthermore, I demonstrate that the regression adjustment, which is largest for lower correlations, is the direct source of the lack of validity of the RDM procedure. Nevertheless, RDM can be considered a valid method to measure an achievement component that is unrelated to intelligence.