The Implications of Teacher Selection and the Teacher Effect in Individually Randomized Group Treatment Trials

In some experimental evaluations of classroom-level interventions it is not practically feasible to randomly assign teachers to experimental conditions. Given such restrictions, researchers may randomly assign students to experimental conditions and consider the teacher to be a part of the intervention. However, in an individually randomized evaluation of a classroom-level intervention, unless teachers are randomized to experimental conditions, it will not be clear whether differences in outcomes between program and control group students are a result of the core components of the intervention or the teachers (i.e., teacher effects). This article clarifies the correct interpretation of "program impacts" when this study design is used. In addition, using the magnitude of estimated teacher effects from past research, this article demonstrates that, if teachers are not randomly assigned to experimental conditions, it is difficult to establish whether the program works or whether the types of teachers selected to teach in program classrooms are simply more or less effective than their control group counterparts. The significant implications of the correct causal interpretation are discussed, and the limitations of this research design are explored