Dear Qiime community,

After spending hours reading about it, and lacking of knowledge about GLM, I am still struggling about the proper way to design model formulas to run Adonis.

Especially, I’m facing two questions :

- What factors should be included in the formula ?
- When to use interaction terms ?

Answers based on practical cases would be greatly appreciated

I’ll illustrate my point with a practical example. I have an experiment with treated and control samples, that were collected at two different months. I also have a bunch of other metadata. The PCoA clearly

shows that samples are clustered according collection month. But, what I am interested in, is the treatment effect.

Is the proper way to run the test is using the formula “Collection_month + Treatment” ? And then, what about other factors that may have an impact on the clustering (I’ve seen that the p-value changes when adding factors) ?

And what about the “Collection_month * Treatment” formula ? I think I understood that it will add the combined effect of month collection and treatment to the model, but I’m not really sure

how to interpret that.

Cheers.