[SciPy-User] Mann-Whitney U Test

[Steve Ramage]

Thanks,

That makes sense, my point with the example was only to demonstrate that the lower median is not necessary the 'lower distribution'.

Cheers,

Steve Ramage




-----Original message-----
From:	josef.pktd at gmail.com
Sent:	Tue 16-12-2014 16:03
Subject:	Re: [SciPy-User] Mann-Whitney U Test
To:	SciPy Users List <scipy-user at scipy.org>; 
> On Tue, Dec 16, 2014 at 8:32 AM, Steve Ramage <sjr at sjrx.net 
> <mailto:sjr at sjrx.net> > wrote:
> Hello,
> 
> I am wondering why the Mann-Whiney U Test always returns the smallu value in 
> the source code, it seems like it should return the U statistic for x or y 
> consistently. 
> https://github.com/scipy/scipy/blob/v0.14.0/scipy/stats/stats.py#L3943 .
> 
> I might be missing something, but it seems impossible to determine which 
> distribution in the lower, 
> 
> 
> smallu is returned because it is is the actual test statistic for comparison 
> with tables (or the normal approximation)   (see Wikipedia)
> 
> However, except for inertia and backwards compatibility it would have been 
> better to change this.
> Also, the function still returns the one-sided p-value which is inconsistent 
> with returning a two-sided test statistic.
> In comparison, ttests return one-sided test statistic and two-sided p-values.
> 
> 
>  
> Wikipedia gives an example of where the lower median is not correct, reproduced 
> below:
> 
> #!/usr/bin/env python
> import scipy.stats as stats
> import scipy.stats.mstats as mstats
> 
> Hare =     [ 1 ,  2,  3,  4,  5,  6,  7,  8,  9, 20, 21, 22, 23, 24, 25, 26, 
> 27, 28, 29]
> Tortoise = [ 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 30, 31, 32, 33, 34, 35, 
> 36, 37, 38]
> 
> print "Hare Median: %d, Tortoise Median: %d, U1: %d, U2: 
> %d"%(mstats.mquantiles(Hare,[0.5])[0],mstats.mquantiles(Tortoise,[0.5])[0], 
> stats.mannwhitneyu(Hare, Tortoise)[0], stats.mannwhitneyu(Tortoise,Hare)[0])
> 
> Hare Median: 20, Tortoise Median: 19, U1: 100, U2: 100
> 
> What's your point?
> 
> smallu is not directly related to the medians.
> 
> We replicate the formulas and results of Wikipedia, so everything looks right 
> to me.
> I don't really understand the example, since I don't remember the details of 
> Mann-Whitney-U or cannot figure them out right now .
> 
> Josef
>  
> _______________________________________________
> SciPy-User mailing list
> SciPy-User at scipy.org <mailto:SciPy-User at scipy.org> 
> http://mail.scipy.org/mailman/listinfo/scipy-user 
> <http://mail.scipy.org/mailman/listinfo/scipy-user> 
> 
> 
> _______________________________________________
> 
> SciPy-User mailing list
> 
> SciPy-User at scipy.org
> 
> http://mail.scipy.org/mailman/listinfo/scipy-user
> 
> 
> 
>