1 2 Model Identifications: Underidentified, Just-identified & Over-identified 11 11 X1 X2 11 11 X1 X2 X3 11 11 X1 X2 X3       X4             1 23 4 5 678 34 1 2 3 4 5 6 Parameter estimated = 4Parameter estimated = 6 Parameter estimated = 8