We don't just add up all of the data points. We then have to divide by the number of data points there are. So we then have to divide, we then have to divide by the number of data points that there actually are. So this might look like very fancy notation, but it's really just saying, add up your data points and divide by the number of data points you have. And this capital Greek letter, sigma, literally means sum, sum all of the x i's from x sub one all the way to x sub n, and then divide by the number of data points you have. Now let's think about how we would denote the same thing, but instead of for the sample mean, doing it for the population mean. So the population mean, they will denote it with mu.