OK, I get it now.  Thank you very much!

Cheers,
Tom



On Tue, Sep 8, 2009 at 8:20 AM, Meyners,Michael,LAUSANNE,AppliedMathematics
<Michael.Meyners@rdls.nestle.com> wrote:

> See the respective help files. The continuity correction only affects
> the normal approximation in wilcox.test. With this small samples sizes,
> the default evaluation is exact, so it doesn't change anything. In
> contrast, kruskal.test is incapable to compute exact values but always
> uses the chi-square approximation. So the discrepancy is between exact
> test and approximation (guess you'd be better off with the former).
>
> If you get the urge to reproduce the p value from kruskal.test using
> wilcox.test, and maybe to better understand what's happening, try
>
> a <- wilcox.test(x1, x2, paired=FALSE, exact=FALSE, correct=FALSE)
>
> (and yes, now with exact=FALSE, the continuity correction makes a
> difference).
>
> HTH, Michael
>
> > -----Original Message-----
> > From: r-help-bounces@r-project.org
> > [mailto:r-help-bounces@r-project.org] On Behalf Of David Scott
> > Sent: Dienstag, 8. September 2009 07:02
> > To: Thomas Farrar
> > Cc: r-help@r-project.org
> > Subject: Re: [R] Equivalence of Mann-Whitney test and
> > Kruskal-Wallis test with k=2
> >
> > Thomas Farrar wrote:
> > > Hi all,
> > >
> > > The Kruskal-Wallis test is a generalization of the two-sample
> > > Mann-Whitney test to *k* samples.  That being the case, the
> > > Kruskal-Wallis test with *k*=2 should give an identical
> > p-value to the Mann-Whitney test, should it not?
> > >
> > > x1<-c(1:5)
> > > x2<-c(6,8,9,11)
> > > a<-wilcox.test(x1,x2,paired=FALSE)
> > > b<-kruskal.test(list(x1,x2),paired=FALSE)
> > > a$p.value
> > > [1] 0.01587302
> > > b$p.value
> > > [1] 0.01430588
> > >
> > > The p-values are slightly different (note that there are no ties in
> > > the data, so computed p-values should be exact).
> > >
> > > Can anyone explain the discrepancy?  It's been awhile since
> > I studied
> > > nonparametric stats and this one has me scratching my head.
> > >
> > > Many thanks!
> > > Tom
> > >
> >
> > The continuity correction? It is true by default for
> > wilcox.test and is not apparent in the help for kruskal.test.
> >
> > David Scott
> >
> > --
> > _________________________________________________________________
> > David Scott   Department of Statistics
> >               The University of Auckland, PB 92019
> >               Auckland 1142,    NEW ZEALAND
> > Phone: +64 9 923 5055, or +64 9 373 7599 ext 85055
> > Email:        d.scott@auckland.ac.nz,  Fax: +64 9 373 7018
> >
> > Director of Consulting, Department of Statistics
> >
> > ______________________________________________
> > R-help@r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide
> > http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> >
>

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