So our best estimate of this standard deviation is 0.05, or you could even view it as 5%. Now, what I want to do is come up with an interval around this sample mean where I'm reasonably confident, using all of my estimates and all of that, where I'm reasonably confident that there's a 95% chance that the true mean is within two standard deviations. Or let me put it this way. There's a 95% chance that the true mean is in that interval. So let me write this down. I want to find an interval such that I am reasonably confident, and I'm putting this kind of touchy-feely language over here, because it's all around the fact that I don't know for a fact that this standard deviation is 0.05. I'm just estimating.