Refering to the notes page 26 :

http://pages.stern.nyu.edu/~gsimon/P...TION7APR08.pdf

it states "This suggests curvature in the relationship with DELEV. Suppose that we create variable D2 = DELEV × DELEV."

but from what i see, it is more of a linear (since straight lines) relationship than a curvature. any comment ?