# 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.

#### Explanation:

Only use a pooled standard deviation if you **KNOW** that the standard deviations for both populations **are equal** (

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