In a test of the effectiveness of a device that is supposed to increase gasoline mileage in automobiles, 12 cars were run, in random order, over a prescribed course both with and without the device in random order. The mileages (mpg)

are given in Table 5.15. Is there evidence that the device is effective?

Table 5.15 Data for Exercise 8 Car No. Without Device With Device 1 21.0 20.6 2 30.0 29.9 3 29.8 30.7 4 27.3 26.5 5 27.7 26.7 6 33.1 32.8 7 18.8 21.7 8 26.2 28.2 9 28.0 28.9 10 18.9 19.9 11 29.3 32.4 12 21.0 22.0 LO.1