Solved – How should I treat F test if one of variances is equal 0

I have two groups with 20 elements in each. In one group variance is equal 0. I want to do f-test. Can I? And how should I interpret results (in this case)?

Since one group's elements are identical values (variance zero), and the other's are not, intuition says that variance is not the same. There is no need for a test.

The F-test for variances takes the ratio of the sample variances: $$ F = frac{S_X^2}{S_Y^2}$$ So you see that if $Y$ is the one group with the identical values (low variance) it is not defined and if $X$ (zero=low variance) it is zero (test failure). So, by definition, the larger variance should be placed in the numerator. Hence, you get an F-statistic of infinity and you can claim that the variances are different.

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