It was more that just statistically significant. The score on the SAT Writing section in isolation explained 17% of variance in cummulative GPA, which was the 2nd strongest correlation with cumulative college GPA in isolation among any of the analyzed criteria, after HS GPA. The combination of all sections of SAT with optimally selected weighting (different from combined score) was no doubt had a noteworthy correlation with college GPA in isolation, probably with a slightly higher % variance explained than the UC study reported. Yet when the SAT removed from their model, they found that no significant loss in predictive ability. The predictive ability of the SAT appeared to be almost entirely duplicated by the other factors that that remained for test optional applicants, particularly the combination of HS GPA + strength of schedule + AP credit total.
That is SAT score in isolation without considering anything else in the application. It would be relevant if colleges admitted based on SAT score in isolation, without considering any other factors. However, for the purposes of test optional admission, the more relevant question is how much of that 21% is duplicated by other components that will be considered among test optional admits? I expect that the overwhelming majority of 21% is duplicated, such that little predictive power will be lost with SAT removed, like occurred in the Ithaca study. Strength of curriculum including having past success in college level classes (taken during HS) is particularly relevant for predicting freshman GPA, yet strength of schedule and/or success in college level classes (taken in HS) does not appear to be considered in the study. I expect SAT scores serves a partial substitute for these missing important criteria.
Itâs also worthwhile to note that colleges rarely emphasize trying to admit the class that will have the highest predicted first-year GPA prior to effects of the a curve. Instead they are generally more concerned with success beyond just freshman year, particularly successfully graduating from the college.