Solved – Is it ok to use control variables in a moderator/interaction analysis in experimental design

controlling-for-a-variableinteractionmediation

I have a question concerning the inclusion of control variables into my research. I have data from an experimental, randomly distributed social protection program.
I want to test whether a moderating variable influences the relationship between treatment and outcome. Also, I want to test for other confounds. However, the experimental design should eliminate the effect of all third variables, right? But in this case, I want to focus on the moderating variable, not the treatment itself. Is it ok to include control variables or is it rather a mediated moderation?

Thanks,
m

Best Answer

It is not "forbidden" to enter further control variables to a model with interaction terms. It just makes the model larger (more complex). What is referred to as mediated moderation or moderated mediation is just a certain linear model (see e.g. here). Whether this model represents your theoretic beliefs can only be judged by you but I strongly recommend the books by Andrew Hayes on that.

Independently from the exact model you are aiming for, you can enter further variables. (If you see it from a path model perspective, you could even decide which of the variables you want to control for your control variables.) You only slightly change the interpretation of your model in that all coefficients of the moderated describe the imaginary case that the other variables are held constant.

You mainly need to consider this when you illustrate your interaction effect. That is, when you plot your conditional regression coefficients you should also state which level of the control variables you conditioned on (typically "mean in all control variables").

Hope this helps - feel free to ask further clarification questions so I can optimize my answer.

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