Four different railroad-track cross-section configurations were tested to determine which is most resistant to breakage under use conditions. Ten miles of each kind of track were laid in each of 5 locations, and the number of cracks and other fracture-related conditions (y) was measured over a two-year usage period. To compare these track designs adequately, however, it is necessary to correct for extent of usage \((x)\), measured in terms of the average number of trains per day that ran over each section of track. Use the following experimental results to test ( 0.01 level of significance) whether the track designs were equally resistant to breakage and to estimate the effect of usage on breakage resistance.

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Track design A Track Track design B design C Track design D x y x y x y x y 10.4 3 16.9 8 17.8 5 19.6 9 19.3 7 23.6 11 24.4 9 25.4 8 13.7 4 14.4 7 13.5 5 35.5 16 7.2 0 17.2 10 20.1 6 16.8 7 16.3 5 9.1 4 11.0 4 31.2 11