The effects of class size on academic achievement have been studied for decades. Although the results of small scale randomized experiments and large-scale econometric studies point to positive effects of small classes, some scholars have seen the evidence as ambiguous. This paper reports analyses of a 4-year, large-scale randomized experiment on the effects of class size, project STAR in Tennessee. Although implementation was not perfect, these analyses suggest that shortcomings in implementation probably led to underestimates of the effects of class size. The analyses reported here suggest class size effects that are large enough to be important for educational policy and that are quite consistent across schools. Thus, small classes appear to benefit all kinds of students in all kinds of schools.
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