The rank-sum test is sometimes thought of as a test for population medians. Under the assumptions of equal spread and shape, the means of two populations will differ if and only if the medians differ; therefore tests for equality of population means are also tests for equality of population medians. This exercise illustrates that when these assumptions are seriously violated, the rank-sum test can give misleading results concerning the equality of population medians. Consider the following two samples: X: 2 3 4 5 6 20 40 50 60 70 80 90 100 Y: -10 -9 -8 -7 -6 -5 -4 20 21 22 23 24 25 26 27 Show that both samples have the same median. Compute the P-value for a two-tailed rank-sum test. If small P-values provide evidence against the null hypothesis that the population medians are equal, would you conclude that the population medians are different? Do the assumptions of the rank-sum test appear to be satisfied? Explain why or why a. b. C. not.

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The rank-sum test is sometimes thought of as a test for population medians. Under the
assumptions of equal spread and shape, the means of two populations will differ if and only
if the medians differ; therefore tests for equality of population means are also tests for
equality of population medians. This exercise illustrates that when these assumptions are
seriously violated, the rank-sum test can give misleading results concerning the equality of
population medians. Consider the following two samples:
X:
2
3 4 5 6
20 40 50 60 70 80 90 100
Y:
-10 -9 -8 -7 -6 -5 -4
20 21 22 23 24 25 26
27
Show that both samples have the same median.
Compute the P-value for a two-tailed rank-sum test. If small P-values provide
evidence against the null hypothesis that the population medians are equal, would you
conclude that the population medians are different?
Do the assumptions of the rank-sum test appear to be satisfied? Explain why or why
a.
b.
C.
not.
Transcribed Image Text:The rank-sum test is sometimes thought of as a test for population medians. Under the assumptions of equal spread and shape, the means of two populations will differ if and only if the medians differ; therefore tests for equality of population means are also tests for equality of population medians. This exercise illustrates that when these assumptions are seriously violated, the rank-sum test can give misleading results concerning the equality of population medians. Consider the following two samples: X: 2 3 4 5 6 20 40 50 60 70 80 90 100 Y: -10 -9 -8 -7 -6 -5 -4 20 21 22 23 24 25 26 27 Show that both samples have the same median. Compute the P-value for a two-tailed rank-sum test. If small P-values provide evidence against the null hypothesis that the population medians are equal, would you conclude that the population medians are different? Do the assumptions of the rank-sum test appear to be satisfied? Explain why or why a. b. C. not.
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