In the following system of equations, the Ys are endogenous, while the Xs are exogenous (with time subscripts suppressed). Determine whether each equation is overidentified, underidentified, or just identified.

(7.27)

(7.28)

(7.29)

In this case, there are G = 3 equations and 3 endogenous variables.

Thus, if the number of excluded variables is exactly 2, the equation is just identified. If the number of excluded variables is more than 2, the equation is overidentified. If the number of excluded variables is less than 2, the equation is not identified.
The variables that appear in one or more of the three equations are Y1 , Y2 , Y3 , X1 , X2 . Applying the order condition to equations (7.27)–(7.29):
Equation (7.27): contains all variables, with none excluded, so that it is not identified Equation (7.28): has variables Y1 and X2 excluded, and so is just identified Equation (7.29): has variables Y1 and X1 excluded, and so is also just identified