Why is the Mann–Whitney U test significant when the medians are equal?

I’ve received a results from a Mann-Whitney rank test that I don’t understand.
The median of the 2 populations is identical (6.9). The uppper and lower quantiles of each population are:

  1. 6.64 & 7.2
  2. 6.60 & 7.1

The p-value resulting from the test comparing these populations is 0.007. How can these populations be significantly different? Is it due to the spread about the median? A boxplot comparing the 2 shows that the second one has far more outliers than the first.
Thanks for any suggestions.


FAQ: Why is the Mann-Whitney significant when the medians are equal?

Source : Link , Question Author : Mog , Answer Author : Harvey Motulsky

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