x1 = 18, n1 = 40, x2 = 30, n2 = 40; 80% confidence interval
a. Use the two-proportions plus-four z-interval procedure as discussed on page 467 to find the required confidence interval for the difference between the two population proportions.
b. Compare your result with the corresponding confidence interval found in parts (d) of Exercises 11.82–11.87, if finding such a confidence interval was appropriate.