# Comparisons of two means from two different groups based on test-statistics

I want to confirm something, i have to compare two means(using test statistics) and if i am not wrong and what i learnt is right then i must use T-test when sample size is less than 30 and population standard deviation is unknown(estimating the population SD with sample SD) otherwise Z-test. But if sample size is greater than 30 but SD is unknown, should i use T-test? I think i should use T-test because if sample size is greater than 30 , still it is not large enough to estimate the SD exactly. Will someone please confirm this ?

• I think you will find that if you do a $t$-test and a $z$-test on the same dataset with a large $n$ the results are more or less indistinguishable. – mdewey Dec 3 '16 at 13:24