The validity of most studies in education, in fact, is subject to doubt because they are seldom replicated. For this reason, these studies constitute an almost unique and particularly useful series in the research literature on school size. It is the only series of studies that focuses on the hypothesis that size mitigates the influence of poverty. The studies included virtually every school in each state studied. Excellence was measured in terms of effect size at varying levels of school socioeconomic status.
Effect size is simply the ratio of an existing difference in terms of standard deviation units.
An effect size of -0.5 is a substantial negative difference, meaning that larger school size would likely decrease achievement . Then, the researchers computed the correlations between achievement and socioeconomic status within the groups. That is, all else equal, larger school size benefits achievement in affluent communities, but it is detrimental in impoverished communities. These schools serve 89 percent of Ohio's ninth graders. The effect was statistically significant only in grade 4.