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Does anyone know a way to calculate the effect size of a Fisher's exact test? Does that even make sense?

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1 Answer 1

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Fisher's exact test is usually used for 2x2 tables. In such a table, you have a few choices for effect size.

One is the odds ratio, which can be calculated by hand pretty easily. If the four cells are $A_{11} A_{12} A_{21} $ and $A_{22}$ then the OR is $A_{11}*A_{22}/A_{21}*A_{12}$.

Another is the difference in proportions in the two rows or two columns: $A_{22}/(A_{12}+A_{22}) -A_{21}/(A_{21}+A_{11})$ or vice versa.

There are others; see any good book on categorical data analysis, such as Agresti's Categorical Data Analysis.

If you have a table that is bigger than 2 x 2, things get more complex, and it depends on whether either row or column is ordinal and on what you are interested in, as well.

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