Interaction terms with random slopes

Perhaps because I never had to deal with software that relies on
explicit nesting of levels, I find it very hard to follow the level-1
and level-2 terminology. Can you provide an example of the model you're
thinking of and how you would test the corresponding hypothesis (e.g.
model comparison, etc.)?

The usual rule of thumb is that for any random slope, you should have
the corresponding fixed-effect slope in the model. There's a discussion
of this over on CrossValidated:
https://stats.stackexchange.com/a/339859/26743

Now, I can think of exceptions to this rule, much like I can think of
exceptions to the rule that you should always include the intercept in a
linear model. But in both cases, if you have to ask, it usually means
you shouldn't do it.

Hi,

I have the following question:

I estimate a random intercept-random slope model. For my research question,
I want to interact the random slope variable with another level-1
variable(which is not necessary as a main effect in the model). I am
interesting in observing whether this level-1 variable moderates the random
slope on the dependent variable.

Do I have to include the level-1 variable as a main effect in the model (as
is required by some linear modelling literature)? I would prefer not to
include this level-1 variable as a main effect due to problems of
multicollinearity.

Thank you very much!

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