What is the Tradeoff between Smaller Classes and Teacher Quality?

This paper investigates the effects of California's class size reduction program on teacher quality and student achievement in an effort to gain a comprehensive understanding of the impact of a large-scale decrease in class size. It uses year-to-year differences in class size generated by natural variation in enrollment and the state's class size reduction program to identify both the direct effects of class size reduction and accompanying changes in teacher quality. The results show that, all else equal, smaller classes raise third-grade mathematics and reading achievement, particularly for lower-income students. However, the expansion of the teaching force required to staff the additional classrooms appears to have led to a deterioration in average teacher quality in schools serving a predominantly black student body. This deterioration partially or, in some cases, fully offset the benefits of smaller classes, demonstrating the importance of considering all implications of any policy change