Assessing the external validity of the ACT as a predictor for intelligence quotient scores

In their 2008 study, Koenig, Frey, and Detterman found a strong predictive relationship between the ACT and a measure of general intelligence derived from the Armed Services Vocational Aptitude Battery (ASVAB). However, their proposed IQ proxy equation was estimated on the pre-1990 revision combined ACT English and Math sections, which will result in less accurate IQ estimates with newer ACT scores. The current study sought to address this limitation by proposing two revised IQ proxy equations, allowing for more accurate IQ estimates with the current iteration of the ACT. Koenig et al.'s original regression formula was also re-estimated when controlling for the effects of age in the measure of intelligence, revealing a stronger predictive relationship between the ACT and the ASVAB's general factor than previously reported. Furthermore, both revised models generalized well on unseen data, suggesting that the ACT has high external validity as an IQ proxy and can be used in research settings for this purpose. A method to correct for systematic prediction error in external regression model validation is also implemented and discussed.