It is important to the success of an experiment like this one that assignment to each of the treatment levels be independent of any other variable that might affect student test results. If for example, teachers in the small classes happened to be more experienced than the teachers assigned to both types of regular classes, we would not be able to separate the effects of teacher experience and class size on student test performance. The problem arises because we suspect that students in much smaller classes are likely to perform better and students taught by more experienced teachers are likely to do better as well. That is the association between class size and teacher experience will amplify the difference in test scores between small and regular classes. See if you can justify the statement: if less experienced teachers are more likely to be assigned to small classes, the relationship of class size with teacher experience will tend to attenuate differences in test score results. I want you to the distributions of kindergarten teachers' experience by class- room type. Use Stata to report the summary statistics for the variable totexpk by cltypek. The latter is a categorical variable that indicates the type of class that a student was assigned to in kindergarten. The categories are identified as follows: 1 (Small class), 2 (Regular), 3 (Regular with aide). Do you see anything that might raise some concern about identifying the effect of class size when you compare the distributions of experience by type of class?