The analogue of a regression line is a regression plane, which has an equation that looks like Y = aX + bZ + c. We can easily compute a, b, c from the data. Here something wonderful happens, which Galton did not realize but Karl Pearson and George Udny Yule certainly did. The coefficient a gives us the regression coefficient of Y on X already adjusted for Z. (It is called a partial regression coefficient and written rYX.Z.)
