So let's put that as the denominator. Let's put that as the denominator, the total variation y, which is the squared error from the mean of the y's. Now we want to know what percentage of this is described by the variation in x. Now what is not described by the variation in x? We want how much is described by the variation in x. But what if we want how much of the total error, how much of the total variation is not described by the line over here, is not described by the regression line. How much of the total data is not?