Four different brands of brake shoes have been installed on 12 city transit buses, with each brand installed on 3 buses selected at random from the 12. The number of thousands of miles before the lining required replacement was as follows for each bus. (Use data file XR12090.)

a. From the variation between the samples, what is the estimated value of the common variance, σ2, that is assumed to be shared by the four populations?
b. From the variation within the samples, what is the estimated value of the common variance, σ2, that is assumed to be shared by the four populations?
c. At the 0.01 level of significance, could the four types of brake linings be equally durable?

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Brand of Brake Lining 23 20 17 23 25 30 4 17 13 19 19 16