Cell values in Chi-square and Fisher's Exact test

Hi, I'm deciding on which test to use to calculate the sample size in g*power. I'm confused about the cell value requirement of the chi-square test of independence. It says that each cell must be at least 5. If it has less than that Fisher's exact test must be used. My study is a 4x8 test of independence. I don't have the data yet so I don't know what the distributions will be like. Each cell might have more than 5 or may even have 0. However, Fisher's exact should only have 2 options (which mine have 4 and 8). So I'm really confused about which test to use. Thanks in advance!


TS Contributor
It says that each cell must be at least 5.
No. The EXPECTED number of observations must be >= 5. It depends on the null
hypothesis, and your assumptions, and/or the study design (e.g. experimental study
with groups of equal size).

For example, if one has 2 x 4 table, including a binary outcome with expected probabilites
0.5 / 0.5, and four experimental groups with proportions 0.25 / 0.25 / 0.25 / 0.25, and a
null hypothesis of independence between group and outcome, then the expected proportion
in each cell would be 0.25 * 0.5 = 0.125. Now it is easy to calculate the total number
of observations required in order to achieve >=5 expected frequencies in each cell.

With kind regards

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