I need to estimate the sample size for a new study with a between-within subject design (4 groups and 2 within factors). In order to do that, I want to perform a power analysis in g-power. I can infer the "possible" effect size of my study from the effect size found in two similar studies already published (I don't have any pilot study do to that unfortunately). The problem is that one study has a within subject design while the other one is a between subject study with two groups. If I am not wrong, the variance, and so the effect size, is different in within vs. between subject design. What can I do to assume a meaningful effect size for my design?
Any idea or suggestion would be really helpful.