For a research proposal, I'm using a 2x2 design. I have 2 independent variables, each with 2 levels, and one dependent variable.

I have 2 hypothesis so far (main effect for each IV) but am unsure about the third hypothesis. The first 2 hypotheses are based on previous research so that's straightforward enough but I don't know how to go about the third hypothesis that deals with the interaction effect between the two.

I'm also not clear on the "within" vs "in-between" part of the design. Is it one or the other for the whole proposal or does the "within groups" refer to the main effect of each IV and the "in-between" refer to the interaction effect?

Grateful for any guidance.