I ran a Generalized Linear Mixed Model in R with 2 centered predictors and a Group variable.

fit1a <- lmer(DV ~ IV1Group + IV2Group + (1|…), data)

One of the predictors interacted signifcantly with the Group variable, in line with my hypothesis. However, literature suggests that this effect may be moderated/mediated by another variable (which happens to often co-occur with the Group variable). Therefore, I included this (centered) variable in another model:

fit1b <- lmer(DV ~ IV1Moderator + IV1Group + IV2Moderator + IV2Group + (1|…), data)

I did the same procedure with a different DV (=2 more models, fit2a and fit2b)

1. When I entered the moderator variable (fit1b), I did not oberserve any sig. effects at all in the second model (the interaction effect I found in fit1a became n.s. as well).

2. In fit2a vs. fit2b entering the presumed moderator resulted in a “new” sig. interaction between the IV1 and Group which I could not observe in the model without the moderator.

How would I interpret these findings? It is relevant for my hypothesis, whether the effect of the IV is linked with the Group variable directly or the Moderator variable. Both results would be interesting for future research.