I want to get the Hansen-Sargan test, after 3SLS, how to do it ? equations: Y1=F(Y2, Y3, X1, X2, X3, X4, marriage,i.edu, i.region) Y2=F(Y1, Y3, Z1, Z2, Z3, marriage,i.edu, i.region) Y3=F(Y1, Y2, W1, W2, Z3, marriage,i.edu, i.region) reg3 (Y1 Y2 Y3 X1 X2 X3 X4 marriage i.edu i.region) (Y2 Y1 Y3 Z1 Z2 Z3 marriage i.edu i.region) (Y3 Y1 Y2 W1 W2 Z3 marriage i.edu i.region) then I write: overid Stata shows: 0: operator invalid