# How to choose an effect size to determine sample size in power analysis for ANOVA?

I am trying to determine sample size using power analysis for ANOVA. That requires the inputs of number of groups, effect size, significance level and power. By default, significance level is 0.05, power is 0.8 and I use 10 groups. The only parameter left is the effect size. I have no idea how much influence one group would have on another so I use 0.2 (small effect) from Cohen (1988) guideline. However, it seems that I might have just made things up. If the 0.2 drops to 0.15 the sample size would go up by 30 observations. Is there a better way to choose an effect size?

• If there are related existing studies in the literature then you might want to look at the effect sizes they found. This could give you a sense of what you might expect to see in your study. Nov 28 '16 at 9:46

• A higher effect size should result less samples I think, I just tested pwr.anova.test in R. Nov 28 '16 at 4:30