Data from the two leading college admissions tests—the SAT and the ACT—can provide a valuable measure of student achievement, but bias resulting from the nonrepresentativeness of test takers is an important concern. The authors take advantage of a policy reform in Illinois that made the ACT a graduation requirement to identify the within- and across-school selectivity of ACT takers. Estimates based on the Illinois policy change indicate substantial positive selection into test participation both across and within schools—at higher-achieving schools, participation rates are higher; within schools, higher-achieving students are more likely to take the test. Despite this, school-level averages of observed admissions test scores are extremely highly correlated with average latent scores. As a result, in most contexts the use of observed mean test scores in place of latent means understates the degree of between-school variation in average achievement but is otherwise unlikely to lead to misleading conclusions.
