This paper analyzes data on 11,600 students and their teachers who were randomly assigned to
different size classes from kindergarten through third grade. Statistical methods are used to
adjust for non-random attrition and transitions between classes. The main conclusions are: (1)
on average, performance on standardized tests increases by 4 percentile points the first year
students attend small classes; (2) the test score advantage of students in small classes expands by
about one percentile point per year in subsequent years; (3) teacher aides and measured teacher
characteristics have little effect; (4) class size has a larger effect for minority students and those
on free lunch; (5) Hawthorne effects were unlikely.