5 Confederate Express Service is attempting to determine how its shipping costs for a month depend on the number of units shipped during a month. For the last 15 months, the number of units shipped and total shipping cost are given in Table 24 (file Ship.xls).

a Determine a relationship between units shipped and monthly shipping cost.
b Plot the errors for the predictions in order of time sequence. Is there any unusual pattern?
c We have been told that there was a trucking strike during months 11–15, and we believe that this may 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?

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TABLE 23 Price Units Sold $1 1,145 $2 788 $3 617 $4 394 $5 275 $6 319 $7 289 $8 241 $9 259 $10 176 $11 179 $12 232 $13 183 $14 181 $15 222 $16 212 $17 186 $18 110 $19 183 $20 172