The Mann-Whitney test uses a normal approximation method to determine the p-value of the test.
is approximately distributed as a normal distribution with a mean of 0 and a standard deviation of 1, N(0,1).
If your data contains ties, then, Minitab adjusts the p-value by replacing the denominator of the above Z
The adjusted p-value is usually more accurate than the unadjusted p-value. However, the unadjusted p-value is the more conservative estimate because it is always greater than the adjusted p-value for a specific pair of samples.
|W||Mann-Whitney test statistics|
|n||size of sample 1|
|m||size of sample 2|
|η1||median of sample 1|
|η2||median of sample 2|
|i||1, 2, …, I|
|I||number of sets of ties|
|ti||number of tied values in the ith set of ties|