RFC: Mann Whitney U test (Wilcoxon rank sum test)

[zross]

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Description

Perhaps it's possible to include the Mann-Whitney U test. I'm seeing the Kruskal test which is close!

For reference, in R this is:

wilcox.test(var~grp, paired = FALSE)

[zross]

A possible js implementation: https://gist.github.com/gungorbudak/1c3989cc26b9567c6e50