[SciPy-user] help with scipy.stats.mannwhitneyu

josef.pktd@gmai... josef.pktd@gmai...
Thu Feb 5 08:33:05 CST 2009

On Thu, Feb 5, 2009 at 6:37 AM, Wavy Davy <bloodearnest@gmail.com> wrote:
> Hi all
> I am using the mannwhitneyu in the stats module, and I was looking the
> code and I see this notice in the docstring.
> "Use only when the n in each condition is < 20 and you have 2
> independent samples of ranks. "
> Am I reading it correctly that this test should only be used with
> sample sizes less than 20?
> I am not a statistican, more a python coder. I have been pointed and
> this test as a more robust version of the t-test, so forgive my
> ignorance.
> Any help would be much appreciated.
> --
> Simon

I briefly looked at the test, the implementation of the test statistic
is mostly as described in

It seems the test statistic is defined with the opposite sign from the
definition in wikipedia.

The doc string statement "Use only when the n in each condition is <
20", I think should be >20, since the pvalue is based on the
asymptotic distribution, which is only correct in larger samples.

I didn't see any unit tests for this test, but I will try to verify
the results later today.

wilcoxon is a similar test for paired instead of independent samples,
and there the recommendation in the docstring is for N>20.


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