Mann-Whitney test, also known as Wilcoxon-Mann-Whitney test, is a non-parametric test for a difference in central location (median) between two independent samples.
The requirements of the test are:
Data in existing Excel worksheets can be used and should be arranged in a List dataset layout or Table dataset layout. The dataset must contain a continuous scale variable and a nominal/ordinal scale variable containing two independent groups.
When entering new data we recommend using New Dataset to create a new 2 variables (1 categorical) dataset ready for data entry.
To start the test:
Excel 97, 2000, 2002 & 2003:
Select any cell in the range containing the dataset to analyse, then click Analyse on the Analyse-it toolbar, click Compare Groups then click Mann Whitney.
If the dataset is arranged using the list layout:Click Variable and select the dependent variable and click Factor and select the independent variable containing the two groups to compare.
The report shows the number of observations analysed, and, if applicable, how many missing values were excluded. Summary statistics for the ranks of each sample are also shown.
The Mann Whitney statistic and hypothesis test are shown. The p-value is the probability of rejecting the null hypothesis, that the samples have the same median, when it is in fact true. A significant p-value implies that the two samples have different medians.
METHOD When the number of observations in each sample is ≤15 an exact p-value is calculated, based on the assumption of no ties (see ). If a few ties are present (the number of ties is shown next to the p-value) the p-value will be conservative, though if more than 10% of the observations are tied then the p-value is unreliable. For >15 observations a normal approximation, with correction for ties, is used (see ).
If both samples are continuous the median difference and confidence interval can be shown to quantify the difference between the samples in terms that can be practically evaluated.
To show a median difference and confidence interval:
NOTE The Hodges-Lehman method used to calculate the confidence interval can be extremely time-consuming for large sample sizes. If no confidence interval is required, and when working with large samples, leave the confidence level blank and the calculation will be skipped.
METHOD The median difference and confidence interval are calculated using the Hodges-Lehman method (see ).
(click to enlarge)