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I am performing a permanova using a bacterial OTU table (16S-metagenomic data) and I want to examine 2 factors if they affect the composition of my OTUs each of the factors is affecting significantly the bacterial OTU composition and I get for each one of them positive F values (as someone would expect) However, the 2-way interaction between the factors is giving me a negative F-value! I understand that in PERMANOVA it is a pseudo-F but still... what does a negative F value mean?

here is the output:

Two-way PERMANOVA
Permutation N: 9999
Source Sum of sqrs df Mean square F p
Taxonomy 5.4349E08 2 2.7175E08 9.6671 0.0001
Actino 1.4429E08 1 1.4429E08 5.1329 0.0001
Interaction -1.3142E08 2 -6.5711E07 -2.3376 0.0004
Residual 2.8391E09 101 2.811E07
Total 3.3955E09 106

any ideas?

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