I have a simple question. I need to calculate a sample size for a study that will be analyzed with a paired t-test.

One of the input parameters for the calculation, is the standard deviation of the differences. However, I do not know it. What I do know, is the standard deviation of the first group and the standard deviation of the second group.

Each subject will receive two treatments. A continuous measure will be taken after each. I know what is the standard deviation of the measure after the 1st treatment and I know what is the standard deviation of the measure after the 2nd treatment, but the standard deviation of D, I don't know.

SAS proc power allows to enter the standard deviations I do know, but I have no idea how from there they estimate the difference standard deviation. PASS on the other hand (which is a remarkable sample size software) asks directly for the difference standard deviation, which I don't know.

My question is, how do I estimate the difference standard deviation ? Via simulation ?