This may be a silly question, but here it goes:
When modelling data, is it valid to add to the model 4 main effects but only 1 specific interaction between two of the variables? or one should look into all the interactions between the 4 main effects in order to create a valid model?
I commonly see that when researchers add interaction in a model, they usually add all possible interactions (often limiting it to 2-ways).
Thanks for the answers (references are well-appreciated)