The sample of the group would be the 22,000 participants.
9/8 because you multiply 3 × 3 and leave the denominator the same


We have critical points whenever both partial derivatives vanish:



The Hessian is


At (0,0), we have

, so (0,0) is a saddle point.
At (-1, -1), we have

and

, so (-1, -1) is a local minimum.
At (1, 1), we have

and

, so (1, 1) is also a local minimum.