data one;
infile 'c:\website\gohttp\class\4213\sas\table13.1.txt' ;
input p q ps di pf;
lag_q=lag(q);
t+1;
proc print;

* estimating a system of equations using Model ;
* see pages 293-297 in your U.E. ;

proc model;
endogenous p q;
exogenous ps di pf;
parms a1-a4 b1-b3;
eq.demand=q-(a1+a2*p+a3*ps+a4*di);
eq.supply=q-(b1+b2*p+b3*pf);

fit demand supply / 2sls;

* estimating a single equation using 2sls ;

proc syslin data=one 2sls;
instruments ps di pf;
model q=p pf;


/* estimating a system of equations
   that contains predeterimined variables using Model ;
*/

proc model;
endogenous p q;
exogenous ps di pf;
parms a1-a4 b1-b4;
eq.demand=q-(a1+a2*p+a3*ps+a4*di);
eq.supply=q-(b1+b2*p+b3*pf+b4*lag_q);

fit demand supply / 2sls;
instruments _EXOG_ lag_q;

run;