13.2 For the following calculations, use the results in column (3) of Table 13.2.

Consider two classrooms, A and B, with identical values of the regressors in column (3) of Table 13.2, except that:

a. Classroom A is a “small class,” and classroom B is a “regular class.”

Construct a 90% confidence interval for the expected difference in average test scores.

b. Classroom A has a teacher with 6 years of experience, and classroom B has a teacher with 12 years of experience. Construct a 95% confidence interval for the expected difference in average test scores.

c. Classroom A is a small class with a teacher with 6 years of experience, and classroom B is a regular class with a teacher with 12 years of experience. Construct a 95% confidence interval for the expected difference in average test scores. (Hint: In STAR, the teachers were randomly assigned to the different types of classrooms.)

d. Why is the intercept missing from column (4)?