This is how many standard deviations we are above the mean, that is that distance right over there. Now, we usually don't know what this is either. We normally don't know what that is either. And the central limit theorem told us that, assuming that we have a sufficient sample size, this thing right here, this thing is going to be the same thing as the standard deviation of our population divided by the square root of our sample size. So this thing right over here can be rewritten as our sample mean minus the mean of our sampling distribution of the sample mean divided by this thing right here, divided by our population mean divided by the square root of our sample size. And this is essentially our best sense of how many standard deviations away from the actual mean we are. And this thing right here, we've learned it before, is a z-score.