Confederate Express is attempting to determine how its monthly shipping costs depend on the number of units shipped during a month. The file P11_48.xlsx contains the number of units shipped and total shipping costs for the last 15 months.
a. Use regression to determine a relationship between units shipped and monthly shipping costs.
b. Plot the errors for the predictions in order of time sequence. Is there any unusual pattern?
c. You have now been told that there was a trucking strike during months 11 through 15, and you believe that this might have influenced shipping costs. How can the analysis in part a be modified to account for the effects of the strike? After accounting for the effects of the strike, does the unusual pattern in part b disappear?