The file Concrete1 contains the compressive strength, in thousands of pounds per square inch (psi), of 40 samples of concrete taken two and seven days after pouring. (Data extracted from O. Carrillo Gamboa and R. F. Gunst, “Measurement Error Model Collinearities,” Technometrics, 34 (1992): 454– 464.)
a. At the 0.01 level of significance, is there evidence that the mean strength is lower at two days than at seven days?
b. What assumption is necessary about the population distribution in order to perform this test?
c. Find the p value in (a) and interpret its meaning.