Adonis output with or without strata

:

Dear list members,

I am having trouble understanding the output of an adonis model I am
running first with then without the argument ?strata? in the formula. I
have seen this question asked before but no answer were provided.

I would like to compare microbial communities associated with healthy and
diseased reef algae. Healthy and diseased algae have been collected in
different islands. I am more interested in the difference between the
Healthy and Diseased groups, I am not really interested in comparing the
islands but still would like to take the islands into account. From what I
read it looks like a way to do it is to include Island as a random factor
in my model.

Variable: Bray-Curtis dissimilarity matrix (dist)

Fixed factor: Disease (Dis)

Random factor: Island

adonis1<- adonis(dist ~ Dis, strata = Island, data = meta)

adonis2<- adonis(dist ~ Dis, data = meta)



However, I get the exact same output for both models:

                   Df SumsOfSqs MeanSqs F.Model     R2 Pr(>F)

Dis_status  1    0.8934 0.89345  1.9845 0.0397  0.001 ***
Residuals  48   21.6103 0.45021         0.9603
Total      49   22.5038                 1.0000
---


 I tried with a subset and I had the same output besides a different
p-value.



Could someone please explain to me why the outputs are identical?



I am sorry if that sounds trivial but but I don?t really understand why
R2, SumsOfSqs and MeanSqs are exactly identical with or without the strata
argument. I?d like to understand better what?s behind it.


Thanks a lot,

Gaelle








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