Teacher experience and the class size effect — Experimental evidence

We analyze teacher experience as a moderating factor for the effect of class size reduction on student achievement in the early grades using data from the Tennessee STAR experiment with random assignment of teachers and students to classes of different sizes. The analysis is motivated by the high costs of class size reductions and the need to identify the circumstances under which this investment is most rewarding. We find a class size effect only for senior teachers. The effect exists at all deciles of the achievement distribution but is less pronounced at lower deciles. We further show that senior teachers outperform rookies only in small classes. Interestingly, the class size effect is likely due to a higher quality of instruction in small classes.