# Sample size calculation

#### sternchen

##### New Member
Hi all,

if got the a question concerning sample size calculation:
When doing a sample size calculation for a randomized clinical trial for comparing to group means I know the following formula:
$$n= \frac{r+1}{r} \frac{\sigma^2 (Z_{\beta}+Z_{\alpha/2})^2}{difference^2}$$

Now my question: How does the sample size change when I have a non-randomized trial instead of a randomized one? Which formula to use?