# 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.

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@whuber, I've even seen at least one statistical software package where the Mann-Whitney test is there as an "alternative" to what is essentially a two-sample $t$-test with unequal variances. Ouch. – cardinal May 21 '11 at 22:49