So that would give us that probability. So what's the probability of getting that extreme of a result? Now, normally when we've done this in the last few videos, we also do not know what the standard deviation of the population is. So in order to approximate that, we say that the z-score is approximately, or the z-statistic is approximately going to be, so let me just write the numerator over again. Over, we estimate this using our sample standard deviation. We estimate it using our sample standard deviation. And this is OK if our sample size is greater than 30.