A consumer advocate researches the length of life between tw o brands of refrigerators, Brand A and Brand B. He collects data on the longevity of 40 refrigerators for Brand A and repeats the sampling for Brand B. A portion of the data is shown in the accompanying table; the full data set is on the text website, labeled Refrigerator Longevity.

Brand A Brand B 16 16 14 20

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18 17

a. Specify the com peting hypotheses to test w hether th e median length o f life differs between the tw o brands.

b. Calculate the value o f the Wilcoxon rank-sum statistic W.

c. Assume th a t the sam pling distribution o f W is approxim ately normal; calculate the value o f the test statistic Z.

d. W ith α = 0.05, does median longevity differ between the tw o brands? Explain.