Class Size, Ability Group Size, and Student Achievement

This article examines the effects of class size on student achievement in two kinds of classrooms: those in which teachers engage in whole class instruction and those in which students are assigned to ability groups for instruction. It is argued that a class size effect will occur through its impact on the length and quality of instruction and that the magnitude of the effect will differ by the type of instruction. Cross-sectional and longitudinal analyses show that class size does have a negative effect on student achievement in both kinds of classrooms; however, when student race is controlled, the class size effect disappears. In the ability-grouped classes, it is group size rather than class size that is negatively related to achievement. This study suggests that classroom pedagogical practices mediate the effect of class size on student learning.