[R-sig-eco] 2-way adonis (PERMANOVA) incl interaction - how to test for main effects?

[Torsten Hauffe]

I see. Thanks for explaining Jari!

On Tue, 16 Oct 2018 at 14:54, Jari Oksanen <jari.oksanen using oulu.fi> wrote:

>
>
> On 16/10/18 11:23, Torsten Hauffe wrote:
> >   "adonis2(speciesdataset~A*B, by="margin") but then only the effect of
> the
> > interaction is tested."
> >
> > This is not entirely correct.
> > adonis2(speciesdataset~A:B, by="margin") would test the interaction
> alone.
> > ~A*B unfolds to ~A+B+A:B
>
> Well, it was correct: the only **marginal** effect in ~A+B+A:B is A:B (A
> and B are not marginal), and by = "margin" will only analyse marginal
> effects.
>
> Cheers, Jari Oksanen
> >
> > On Tue, 16 Oct 2018 at 11:51, Ellen Pape <ellen.pape using gmail.com> wrote:
> >
> >> Hi all,
> >>
> >> I don't know whether this is the correct mailing group to address this
> >> question:
> >>
> >> I would like to perform a 2-way permanova analysis in R (using adonis in
> >> vegan). By default you are performing sequential tests (by="terms"), so
> >> when you have 2 or more factors, the order of these factors matter.
> >> However, since I wanted to circumvent this, I chose for the option
> >> by="margin" (adonis2(speciesdataset~A*B, by="margin")) but then only the
> >> effect of the interaction is tested. On the "help page" of anova. cca it
> >> says: "if you select by="margin" -> the current function only evaluates
> >> marginal terms. It will, for instance, ignore main effects that are
> >> included in interaction terms."
> >>
> >>
> >> My question now is: can I somehow get the main effects tested anyhow,
> when
> >> the interaction term is not significant?
> >>
> >> Thanks,
> >> Ellen
> >>
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