I am attempting to use relevant effects size from a published study to obtain a suggested sample size for my related study in G*power. However, 1) the previous study only compared 2 groups w/ t-test and I want to compare 5 with one way ANOVA and 2) the suggested sample size (if i was replicating with a t-test) is much larger than the study I gathered the input from.

First question: Should I assume a much lower effect size than obtained with a t-test because I'm adding comparisons?

Second question: Why does G*Power suggest more than double the subjects (than published) when I compute for a simple t-test (the same test they did) using their established effect size? I'm basically just inputing all the numbers from the first study and being told their sample size shouldn't have worked. This finding is making me second guess the suggested Ns when calculating for an ANOVA.

What am I missing? Thank you in advance for any replies.