7.5 A Nonparametric Test for Comparing Two Populations: Independent Samples (Optional)

Suppose two independent random samples are to be used to compare two populations, but the t-test of section 7.2 is inappropriate for making the comparison. We may be unwilling to make assumptions about the form of the underlying population probability distributions, or we may be unable to obtain exact values of the sample measurements. If the data can be ranked in order of magnitude in either of these cases, the Wilcoxon rank sum test (developed by Frank Wilcoxon) can be used to test the hypothesis that the probability distributions associated with the two populations are equivalent.

For example, consider an experimental psychologist who wants to compare reaction ...

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