And this thing right here, we've learned it before, is a z-score. Or when we're dealing with an actual statistic, when it's derived from the sample mean statistic, we call this a z statistic. And then we could look it up in a z-table or in a normal distribution table to say, what's the probability of getting a value of this z or greater? 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.