Mann-Whitney U

Surprise! This is just another name for the Wilcoxon rank-sum test.

Wilcoxon rank sum: do two unpaired populations have the same mean?

• Null hypothesis: there is no significant difference between two groups
• Alternative hypothesis: there is significant difference between the two groups

Here is an example of how your data should be formatted for a Wilcoxon rank sum test:

Height Group 1 (inches)	Height Group 2 (inches)
68	48
70	50
75	60
70	52
72	55
73	58
77	56
78	59
71	51

Values returned from a Wilcoxon test

The Wilcoxon test will produce the following values. I have provided a brief description of how to interpret them.

• Z-score: the number of standard deviations away from the mean out value of interest is.
• P-value: the probability that the results from your sample occurred by random chance. Important note: the p-value does not indicate that the treatment is the CAUSE. All that it states is that the two means are significantly different from each other.
  • P-value < 0.05 indicates the means of each group are different from each other (Reject the null hypothesis)
  • P-value > 0.05 indicates that you cannot conclude that means of each group are different from each other (Fail to reject the null hypothesis)