The tire wear of n 5n 5100 1 2 tires of two types in Example 8.9 had sample means and standard deviations shown in the table. Find an approximate 95% confidence interval for m1, the average tire wear for tire type 1, and a 95% confidence interval for the difference in the tire wear for the two types of tires.

1. Select stat ➤ TESTS and either 7:ZInterval, 9:2-SampZInt… Select Stats in the “Inpt:” line, and enter the values for n1 and x1 (or n1, n2, x1, and x2) on the appropriate lines.
2. Enter the value for s 51200 1 (or s1 and s2) on the line marked “s ” (or s1 and s2). (Notice that we are assuming that the population standard deviations, s1 and s2, can be approximated by s1 and s2.) You can change the confidence level from the default of 95% to another level if necessary. When you move the cursor to Calculate and press enter, the confidence intervals will appear, shown on the screens in Figures 8.14

(a) and 8.14(b).

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Type 1 Sample Mean 26,400 miles Sample Standard Deviation 1200 miles 2 25,100 miles 1400 miles