Air Express, an overnight mail service, is concerned about the operating efficiency of the package-sorting departments at its Toledo, Ohio, terminal. The company would like to monitor the time it takes for packages to be put in outgoing delivery bins from the time they are received. The sorting department operates six hours per day, from 6 P.M. to midnight. The company randomly sampled four packages during each hour of operation during four consecutive days. The time for each package to move through the system, in minutes, is given in the second table on the next page
a. Construct an 
-chart from these data. In order for this chart to be meaningful, what assumption must be made about the variation of the process? Why?
b. What does the chart suggest about the stability of the package-sorting process? Explain.
c. Should the control limits be used to monitor future process output? Explain.
