F or variance ratio test. See here
RonL
A machine dispenses a liquid drug into bottles in such a way that the standard deviation of the content is 81 milliliters. A new machine is tested on a sample of 24 containers and the standard deviation for this sample group is found to be 36 milliliters. At the 0.05 level of significance , test the hypothesis that the new machine produces a lower standard deviation.
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