# Combining standard deviations

I'm wondering how to combine the SDs for two results. I have the standard deviations of the baseline results and the SDs of the change from baseline. I'm wondering how to calculate the SD of the endpoint.

How do I do this?

There is a simple formula for the variance which is just the square of the standard deviatiotn. $$\text{Var}(X + Y) = \text{Var}(X) + \text{Var}(Y) + 2\text{Cov}(X, Y)$$
In your case $X$ is the baseline result and $Y$ is the change. The critical thing is that you also need the covariance, which you did not mention.