Testing the difference between the observations of two groups by using Mann-Whitney Test has given the following output (from minitab):
N Median positives 137 1.0000 negatives 892 1.0000 Point estimate for η1 - η2 is 0.0000 99.0 Percent CI for η1 - η2 is (0.0001,-0.0001) W = 56899.5 Test of η1 = η2 vs η1 ≠ η2 is significant at 0.0000 The test is significant at 0.0000 (adjusted for ties)
When I was testing other pairs of groups, I concluded that a group tends to have larger values than another group based on their medians, if the test shows a statistically significant difference. However, the example above shows that I may not be interpreting the results correctly.
Here is some additional descriptive statistics for the example above:
Variable | Mean | StDev | Minimum | Q1 |Median| Q3 | Maximum positives | 4.13 | 13.17 | 1.00 |1.00| 1.00 |1.00| 116.00 negatives | 6.851 | 20.503 | 0.000 |1.00| 1.00 |5.00| 434.000
- What have I done wrong?
- How can I figure out the direction of the difference? Can I conclude that the negatives tends to have larger values based on their larger Q3, for example?