The IRS is concerned with improving the accuracy of tax information given by its representatives over the telephone. Previous studies involved asking a set of 25 questions of a large number of IRS telephone representatives to determine the proportion of correct responses. Historically, the average proportion of correct responses has been 72 percent. Recently, IRS representatives have been receiving more training. On April 26, the set of 25 tax questions were again asked of 20 randomly selected IRS telephone representatives. The numbers of correct answers were 18, 16, 19, 21, 20, 16, 21,16,17, 10, 25, 18, 25, 16, 20, 15, 23, 19, 21, and 19.
a. What are the upper and lower control limits for the appropriate p-chart for the IRS? Use z = 3.
b. Is the tax information process in statistical control?