Some Findings from an Independent Investigation of the Tennessee STAR Experiment and from Other Investigations of Class Size Effects

Abstract

While random-assignment experiments have considerable conceptual appeal, the validity and reliability of results depends crucially on a number of design and implementation issues. This paper reviews the major experiment in class size reduction-Tennessee’s Project STAR-and puts the results in the context of existing nonexperimental evidence about class size. The nonexperimental evidence uniformly indicates no consistent improvement in achievement with class size reductions. This evidence comes from very different sources and methodologies, making the consistency of results quite striking. The experimental evidence from the STAR experiment is typically cited as providing strong support of current policy proposals to reduce class size. Detailed review of the evidence, however uncovers a number of important design and implementation issues that suggest considerable uncertainty about the magnitude of any treatment effects. Moreover there is reason to believe that the commonly cited results are biased upwards. Ignoring consideration of the uncertainties and possible biases in the experiment, the results show effects that are limited to very large (and expensive) reductions in kindergarten or possibly first grade class sizes. No support for smaller reductions in class size (i.e., reductions resulting in class sizes greater than 13–17 students) or for reductions in later grades is found in the STAR results.