1. For reasonable assumptions if you’re studying an interaction then you might need 16x larger samples—see Gelman. Essentially standard error is double for interactions and Andrew thinks that interaction effects being half the size of main effects is a good starting point for estimates, giving (2×2)2=16 times larger samples.
2. When estimating cohen’s d, it is important that you know whether the study is between or within subjects—within subject studies will give much lower standard error and thus require much smaller samples. Again Gelman discusses.
Additional thoughts:
1. For reasonable assumptions if you’re studying an interaction then you might need 16x larger samples—see Gelman. Essentially standard error is double for interactions and Andrew thinks that interaction effects being half the size of main effects is a good starting point for estimates, giving (2×2)2=16 times larger samples.
2. When estimating cohen’s d, it is important that you know whether the study is between or within subjects—within subject studies will give much lower standard error and thus require much smaller samples. Again Gelman discusses.