Why is a pooled standard deviation used for a two sample t-test?

1 Answer
Nov 12, 2015


Pooled standard deviation is rarely used in hypothesis testing.


Only use a pooled standard deviation if you KNOW that the standard deviations for both populations are equal (#sigma_1=sigma_2#).

Rarely does the statistician know the population standard deviations and even more rare would be that they know that the population standard deviations are equal .

In summary, most two-sample mean tests are done under the assumption that the standard deviations are not equal . Under this scenario, pooled standard deviations are not used.

hope that helps