So we start at the first data point and we go all the way to the capital Nth data point. So every data point we add up. So this is the ith data point. So X sub 1 plus X sub 2 all the way to X sub capital N. And then we divide by the total number of data points we have. Well, how do we calculate the sample mean? Well, the sample mean, we do a very similar thing with the sample. And we denote it with an X with a bar over it.