Welch’s approximation

Where the problem is solely one of unequal SDs, there is a minor variant of the test, Welch s approximate t that does not have this requirement. In fact, in Minitab, this is the default test, and if you want the classical f-test, you will have to select it (see the web site for details). 

Hauschke D, Steinijans VW, Hothorn LA (1996) A note on Welch s approximate t-solution to bioequivalence assessment. Biometrika 83 236-237. 

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