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?

>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,

-- 
Gian

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