multiple factors in vegan:betadisper?

Dear Cameron,

A while ago, I had asked a similar question to Marti Anderson. Here's is
what she had replied:

        "I have actually moved away from doing 2-way PERMDISP types of
        analyses. The reason for this is that there are inherent
        difficulties in testing homogeneity of dispersions across the
        main effects in cases where there is an interaction between the
        two factors in their *locations*, when tested using PERMANOVA.
        Think of it this way - suppose there is no differences in the
        locations of samples in multivariate space among the levels of
        factor A and in level 1 of factor B, but there are such
        differences (once again, in *location*) in level 2 of factor B
        (which would correspond to an interaction in PERMANOVA). Then,
        clearly, this would be detected as a difference in dispersion,
        even though it is entirely due to the locations of cells
        (combinations of factors) in the two-way design. Thus, it seems
        to me that for the two-way design, to test dispersions, one can:

        (i) test for differences in dispersion among the individual axb
        cells (which is simply a one-way test where the individual cells
        are treated as levels of a single factor) and

        (ii) test for an interaction in a two-way PERMANOVA and if this
        is NS, then one can examine the test of dispersions (or
        locations, for that matter) separately and independently for the
        two factors."

Hope this helps.

Cheers

Etienne

Le mardi 21 septembre 2010 ? 13:24 -0700, Cameron N. Carlyle a ?crit :

multiple factors in vegan:betadisper?

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