I have a statistics interpretation question. I've recently performed a two-way anova to identify an interaction term between my categorical independent variables (genotype + temperature) that influences my continuous dependent variable (speed). My hypothesis is that genotype+temperature strongly interact to reduce speed.
In my case, the interaction term is significant - ie., genotype and temperature significantly interact, and I can easily observe that speed is reduced in this case.
I am now directed by common practice to perform a post-hoc test, but I don't really understand why:
What post-hoc test is appropriate to help confirm the interaction between temp and genotype? I am not interested in anything except the interaction, as I consider all of the other conditions to be controls.
That said, is such a post-hoc test necessary, given that the ANOVA itself is designed to (and succeeds) in revealing a significant interaction? Is it necessary to determine the sign of the interaction?
Bonus points if you can point me to a nonparametric test for this, as my data is not gaussian (but my large N=100+ helps me be comfortable with using the data in the ANOVA itself).