# T Test

#### obh

##### Active Member
Hi

T-Test is used instead of Z-Test when we don't know the standard deviation, or when the sample size is too small (less than 31)

When we know the standard deviation but the sample size is small, do we use T-Test with sample standard deviation (S)?
or T-Test with the known standard deviation (sigma)?

Thanks
O

#### GretaGarbo

##### Human
When you know the standard deviation, you do a z-test using the known standard deviation. It doesn't matter if the sample size is small or large.