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Do Low-Achieving Students Benefit More from Small Classes? Evidence from the Tennessee Class Size Experiment

Recent evidence about the effects of class size on academic achievement from randomized experiments points to positive effects of small classes. However, the evidence about the mechanism producing these effects is less clear. Some scholars have argued for mechanisms that would imply greater effects of small classes for low-achieving students. This article investigates possible differential effects of small classes on achievement using data from Project STAR, a four-year, large-scale randomized experiment on the effects of class size. We examined the differential effects of small classes for students in the bottom half and bottom quarter, respectively, of their class's achievement distribution in kindergarten. Although small class effects are somewhat larger for low-achieving students in reading, the differential effects (interactions) are not statistically significant. Moreover, the small class effects for low-achieving students in mathematics are actually smaller than those for higher achieving students. Thus while there are unambiguous positive effects of small classes on achievement, there is no evidence for differentially larger effects of small classes for lower achieving students.