8.4 TWO-STAGE LEAST SQUARES, CONTROL FUNCTIONS, AND LIMITEDINFORMATION MAXIMUM LIKELIHOODThus far, we have assumed that the number of instrumental variables in Z is the same as the number of variables (exogenous plus endogenous) in X. In the typical application, there is one instrument for the single endogenous variable in the equation. The model specification may imply additional instruments. Recall the market equilibrium application considered in Examples 8.1 and 8.4. Suppose this were an agricultural market in which there are two exogenous conditions of supply, InputPrice and Rainfall. Then, the equations of the model are