A four year study in Tennessee randomly assigned children in kindergarten through third grade into classes of different sizes. The results of this experiment indicated that the smaller class sizes improved test scores [Krueger, 1999]. However, about 10 percent of the students in the study switched classes during the course of the experiment. Explain how such switching might lead to biased estimates of the impact of lowering class size. Under what conditions would switching be compatible with unbiased results? Give some examples of reasons for switching that would lead to biased results and some that would not.