So we can calculate it for a certain set of data. Now what I want to do is kind of come up with a more meaningful estimate of how good this line is fitting the data points that we have. To do that, we're going to ask ourselves the question, how much, or we could even say what percentage, what percentage of the variation in y is described by the variation in x? And so let's think about this. How much of the total variation in y, there's obviously variation in y. This y value is over here, this point's y value is over here. There's clearly a bunch of variation in the y, but how much of that is essentially described by the variation in x or described by the line?