I tried finding similar cases and questions but failed and got even more confused because the answers seem to depend on when the questions were asked due to changes in the lme4 package. So I'm sorry if the answer to this question is obvious to some but I just can't wrap my head around it and would appreciate some insights:
Dependent variable: rating data
Independent variable: word_variant (type a vs. type b)
Repeated measures design in which every participant rates every word (fully randomized) and sees both types of a word variant (e.g., type a=shoes vs. type b=slippers).
We also got participants to state their level of education and 2 other between-subject variables. They are not meant as independent variables (fixed effects) but we want to see if they account for variance in the data.
Now I'm unsure where these between-subject control variables go in the model. Can I just include them as fixed effects even though they are not fixed effects in terms of an independent variable? Can I include them in the subject random effect structure even though they are between-subject variables?
Intuitively, I would put the between-subject control variables in the subject and item random slope and not use them as fixed effects:
summary(my_model_1 <- lmer(rating ~ word_variant + (1+word_variant*education| subject) + (1+word_variant*education| item), data))
Alternatively, I could include the between-subject control variables as fixed effects but not in the subject random effect structure:
summary(my_model_2 <- lmer(rating ~ word_variant*education (1+word_variant| subject) + (1+word_variant*education| item), data))
Any insights would be highly appreciated! I hope my question is clear enough. Let me know if more information is required!
Thank you!