From ripley at stats.ox.ac.uk  Wed Jan  2 08:14:56 2008
From: ripley at stats.ox.ac.uk (Prof Brian Ripley)
Date: Wed, 2 Jan 2008 07:14:56 +0000 (GMT)
Subject: [Rd] Error from wilcox.test
In-Reply-To: <477AB642.6070201@ebi.ac.uk>
References: <477AB642.6070201@ebi.ac.uk>
Message-ID: <Pine.LNX.4.64.0801020708590.6430@gannet.stats.ox.ac.uk>

Try exact = TRUE: the default switches to a normal approximation that will 
not be adequate in your extreme example.

On Tue, 1 Jan 2008, Wolfgang Huber wrote:

> When one of the two groups has only one member and the other one more
> than 49, wilcox.test will exit with the below error message,
>
>  n=51; wilcox.test(1:n ~ 1:n==1, conf.int=TRUE)
>
>  Error in uniroot(wdiff, c(mumin, mumax), tol = 1e-04, zq =
> qnorm(alpha/2,  :
>   f() values at end points not of opposite sign
>
>
> whereas with n=50 a result is returned:
>
>  n=50; wilcox.test(1:n ~ 1:n==1, conf.int=TRUE)
>
>         Wilcoxon rank sum test
>
> data:  1:n by 1:n == 1
> W = 49, p-value = 0.04
> alternative hypothesis: true location shift is not equal to 0
> 95 percent confidence interval:
>   1 49
> sample estimates:
> difference in location
>                     25
>
>
> I wonder whether it would be worthwhile to make the wilcox.test function
> handle such (admittedly pathologic) cases more gracefully.
>
>
> Happy New Year to all -
>   Wolfgang
>
> ------------------------------------------------------------------
> Wolfgang Huber  EBI/EMBL  Cambridge UK  http://www.ebi.ac.uk/huber
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595



[Rd] Error from wilcox.test
Prof Brian Ripley