= ''g''_''Y''(''y''), obtained by regressing y on x and x on y respectively. (Here, ? is measured counterclockwise within the first quadrant formed around the lines' intersection point if , or counterclockwise from the fourth to the second quadrant if ''r'' < 0.) One can show that if the standard deviations are equal, then 1=''r'' = sec ''?'' - tan ''?'', where sec and tan are trigonometric functions.

For centered data (i.e., data which have been shifted by the sample means of