But let's say you just had a sample of these random variables. How could you estimate them? Well, if you were estimating it, the expected value, let's say you just have a bunch of data points, a bunch of coordinates, and I think you'll start to see how this relates to what we did with regression. The expected value of X times Y, it can be approximated by the sample mean of the products of X and Y. This is going to be the sample mean of X and Y. You take each of your XY associations, take their product, and then take the mean of all of them. So that's going to be the product of X and Y, and then this thing right over here, the expected value of Y, that can be approximated by the sample mean of Y, and the expected value of X can be approximated by the sample mean of X.