Confederate Express is attempting to determine how its monthly shipping costs depend on the number of units shipped during a month. The file P16_58.xlsx contains the number of units shipped and total shipping costs for the past 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. Suppose there was a trucking strike during months 11 to 15, and we believe that this might have influenced shipping costs. How could the answer to 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? (Hint:

Use a dummy variable.)