On the basis of extensive tests, the yield point of a particular type of mild steel-reinforcing bar is known to be normally distributed with σ = 100. The composition of bars has been slightly modified, but the modification is not believed to have affected either the normality or the value of s.
a. Assuming this to be the case, if a sample of 25 modified bars resulted in a sample average yield point of 8439 lb, compute a 90% CI for the true average yield point of the modified bar.
b. How would you modify the interval in part (a) to obtain a confidence level of 92%?