Heterogeneous Class Size Effects: New Evidence from a Panel of University Students *

Using administrative records from a UK university, we present evidence on the effects of class size on students’ test scores. We estimate non‐linear class size effects controlling for unobserved heterogen‐eity of students and faculty. We find that: ( i ) at the average class size, the effect size is −0.108; ( ii ) the effect size is negative and significant only for the smallest and largest ranges of class sizes and zero in intermediate class sizes; ( iii ) students at the top of the test score distribution are more affected by changes in class size, especially when class sizes are very large.