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:

- 6.64 & 7.2
- 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.

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