In the future, you're not going to have all of the data. You're just going to have some samples of it. You're going to try to estimate things for the entire population. So I don't want you to worry too much about that just now. But if you are going to go further in statistics, I just want to make that clarification. Now, the population mean, or the arithmetic mean, of this data set right here, it is negative 10 plus 0 plus 10 plus 20 plus 30 over, we have 5 data points, over 5. And what is this equal to?