I am going to conduct a pretty simple experiment: one within factor with 8 levels for a single group of participants. I'm trying to determine the apporpriate sample size using GPower based on an effect size of eta square = 0.60 from a previous study.

However, the end sample size is odd. Here are the parameters I entered :

F tests > ANOVA: Repeated measures, within factors > A priori

Effect size = 0.6 => (f = 1.2247449)

alpha = 0.05 / Power = 0.80

Number of groups = 1

Number of measurements = 8

Corr among measures = 0.5

Nonsphericity correction = 1

Result : a sample size of only 2 participants.

What exactly is the problem with my input? Is there another way to compute the appropriate sample size?


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