Grades Best Predict College Success
Sage Journals: "High school GPAs (HSGPAs) are often perceived to represent inconsistent levels of readiness for college across high schools, whereas test scores (e.g., ACT scores) are seen as comparable ... We found students with the same HSGPA or the same ACT score graduate at very different rates based on which high school they attended. Yet, the relationship of HSGPAs with college graduation is strong and consistent and larger than school effects. In contrast, the relationship of ACT scores with college graduation is weak and smaller than high school effects, and the slope of the relationship varies by high school."
"Grades are assigned based on a potentially wide-ranging array of tasks, measured over time, capturing academic knowledge, skills, behaviors, and effort and incorporating teacher judgment ... The fact that grades are based on a wide range of factors with judgment from many different teachers makes them potentially highly variable across contexts. At the same time, the fact that they are based on a large number of raters (teachers) across a wide range of relevant tasks could actually make them very reliable as indicators of academic readiness for college, where students will also be asked to do a range of tasks with different expectations assessed by many different instructors."
"There is no reason to believe a priori that tests would necessarily be more reliable than grades as predictors of college performance. Standardized tests assess students on a narrow range of skills (mostly a subset of what students learn in English and math classes) in one type of condition (a timed test), whereas colleges expect students to have broad knowledge and skills across many subjects and to show consistent effort in different types of assignments over months at a time."