how can two-way interaction be a model why do you need to include main effects in a model? what to do with interactions that don't have main effects which is an important difference between a two dimensional interaction and a two-directional interaction is it ever valid to include a two way interaction in a model without including the main effects which interaction should be included in the main effect of a research proposal? is it ever valid to include an interaction in a model without including the main effects what is the interaction in a model? does a 2 way interaction need to be included can you include the primary and secondary effects when do you include the main effects in the model why not include main effects in a model which interaction would you include in the main effects for a model? what is the main effect of two-way interaction when is it valid to include two ways interaction in a model without excluding the main effects? is an interaction valid in a model why is it valid to include interaction in a model without including the main effects? why is a two way interaction included in a model when to include the interaction and main effect how to include two way interaction in a model