Hi Edi,

Thank you very much. Now I got it.

Gian

On 2 March 2012 20:34, Eduard Szöcs <szoe8822@uni-landau.de> wrote:

> Hai Gian,
>
> On 02/03/12 17:30, Gian Maria Niccolò Benucci wrote:
> > Hi Members,
> >
> > On 24 samples from two different fungal ectomycorrhizal communtites
> > (natural vs. cultivated), I run betadisper() to test multivariate
> > homogeneity of group dispersions.
> > To test if one group is more variable than the other, I run anova() of
> the
> > distances to group centroids. Moreover I tried o run a permutest() to
> > generate a permutation distribution of F under the Null hypothesis of no
> > difference in dispersion between groups.
> > Maybe is a stupid question but I can't explain why with both F and
> > non-parametric statistics I got same results?
> What do you mean, with the same results? The p-values are slightly
> different. In the parametric test, this is assed by the F-Distribution
> [see pf(18.485, 1, 22, lower.tail = FALSE)].
>
> In the Permutationtest, you generate a distribution of F-values by
> shuffling the group vector and compare your original F with this
> generated distribution. Of course the F is the same in both methods,
> only how the p-values is assessed differs.
>
> HTH,
>
> Edi
>
> >
> >> betadisper2_cent<- betadisper(dist, groups, type = c("centroid"))
> >> anova(betadisper2_cent)
> > Analysis of Variance Table
> >
> > Response: Distances
> >            Df  Sum Sq Mean Sq F value    Pr(>F)
> > Groups     1 0.20906 0.20906  18.485 0.0002903 ***
> > Residuals 22 0.24881 0.01131
> > ---
> > Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
> >
> >> permutest(betadisper2_cent, control=permControl(nperm=9999), pairwise=T)
> > Permutation test for homogeneity of multivariate dispersions
> >
> > No. of permutations: 9999
> >
> > **** STRATA ****
> > Permutations are unstratified
> >
> > **** SAMPLES ****
> > Permutation type: free
> > Mirrored permutations for Samples?: No
> >
> > Response: Distances
> >            Df  Sum Sq Mean Sq      F N.Perm Pr(>F)
> > Groups     1 0.20906 0.20906 18.485   9999  3e-04 ***
> > Residuals 22 0.24881 0.01131
> > ---
> > Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
> >
> > Thank you so much,
> >
> >
> >
> > _______________________________________________
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> > R-sig-ecology@r-project.org
> > https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
>
>
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>
>
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>


-- 
Gian Maria Niccolò Benucci, Ph.D.
Department of Applied Biology
University of Perugia
Borgo XX Giugno, 74
06121 - Perugia, ITALY
Tel: +390755856433
Mobile: +393477063881
Email: gian.benucci@gmail.com



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