Whereas the direction of main effects can be interpreted from the sign of the estimate, the interpretation of interaction effects often requires plots. This task is facilitated by the R package sjPlot (Lüdecke, 2022). In Bernabeu (2022), the sjPlot function called plot_model served as the basis for the creation of some custom functions. One of these functions is alias_interaction_plot, which allows the plotting of interactions between a continuous variable and a categorical variable. Importantly, the categorical variable is replaced with an alias variable. This feature allows the back-transformation of the categorical variable to facilitate the communication of the results, for instance, when the categorical variable was sum-coded, which has been recommended for mixed-effects models (Brauer & Curtin, 2018).

Below, we’ll use the function with a model fitted using lmerTest (Kuznetsova et al., 2022), although the function also works with several other models (see sjPlot manual). The plot can be reproduced using the materials at https://osf.io/gt5uf.