I would appreciate if anyone could help me understand the difference between marginal effects and effect modification in
marginal_effects.brmsfit and in emmeans estimation based on a
marginal effects are defined as “instantaneous rates of change: for a dichotomous explanatory variable, a marginal effect shows how predicted probabilities change when the variable changes from 0 to 1” here are elsewhere.
On the other hand,
effect modification here and elsewhere is defined as “when the magnitude of the effect of the primary exposure on an outcome (i.e., the association) differs depending on the level of a third variable”.
brms documentation says “Since we condition on rather than actually marginalizing variables, the name marginal_effects is possibly not ideally chosen in retrospect.”
And the documentation of
emmeans states that " … EMMs are based on a model . Neither of these plots is an interaction plot of the data ; they are interaction plots of model predictions; and since both models do not include an interaction, no interaction at all is evident in the plots."
I fail to see where the overlap between these two measures ends. Specifically, what is the difference between
marginal effects without interaction and
marginal effects with interaction in
brms in terms of their meaning?