13.21 Study time helps CPI? Refer to the previous exercise.

a. Report and interpret the P-value for testing the hypothesis that the population slope coefficient for study time equals 0.

b. Find a 95% confidence interval for the true slope for average daily study time (ADST). Explain how the result is in accord with the result of the test in part a.

c. Does the result in part a imply that in the corresponding population, the average daily study time (ADST) has no association with college CPI? Explain. (Hint: What is the impact of also having HSG and AMAP in the model?)