Power analysis (gpower) - interactions


New Member
I'd like to conduct a power analysis in gpower to determine the total N required in a study, with the following requirements:

power desired: 0.80
alpha: .05
effect size: unsure what to use here (explained below)
study design: 2x2 within subjects anova (4 conditions) where my main hypothesis is that there will be an interaction between the 2 factors. Both factors are within subjects and fixed

if this were a one way design, a t test, or a 2 way where my main concern was main effects then I would know how to do this calculation in gpower. The problem is that I'm expecting an interaction, so I'm not sure how to factor that in and how it may impact the calculation. This also means I'm not sure what an effect size means in an interaction case, and how to decide what a good effect should be for this calculation (have not been able to find similar studies in existing research to borrow an effect size from)