The wearing qualities of two types of automobile tires were compared by road-testing samples of n = n = 100 tires for each type. The number of kilometres until wearout was defined as a specific amount of tire wear. The test results are given in Table 8.4. Esti- mate (u), the difference in mean kilometres to wearout, using a 99% confidence interval. Is there a difference in the average wearing quality for the two types of tires?