•Taken separately, high school GPA is a better predictor of college graduation rates than SAT/ACT score. This findings holds true across institution type, and gets stronger the less selective an institution is. High school GPA is three to five times more important in predicting college graduation than SAT/ ACT score.
•SAT and ACT scores are proxies for high school quality. When the authors factored in which high schools students attended (i.e. high school quality), the predictive power of high school GPA went up, and the predictive power of SAT/ ACT scores fell below zero.
This is a subtle statistical point: unnormalized, ACT scores have predictive power, high scores mean higher college success. However when GPA's are normalized by the schools average ACT score that predictive power disappears. That is pretty strong signal that standardized test scores are an independent measure of "High School Quality", while GPA is a measure of the student's individual performance.
I have been a bit concerned about alingment to the ACT's "College Readiness Standards", but this large study really justifies that move.