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The STATESPACE Procedure 
Instead of allowing the STATESPACE procedure to select the model automatically, you can use FORM and RESTRICT statements to specify a state space model.
The FORM statement specifies the number of lags of each variable to include in the state vector. For example, the statement FORM X 3; forces the state vector to include x_{tt}, x_{t+1t}, and x_{t+2t}. The following statement specifies the state vector ( x_{tt}, y_{tt}, x_{t+1t} ), which is the same state vector selected in the preceding example:
form x 2 y 1;
You can specify the form for only some of the variables and allow PROC STATESPACE to select the form for the other variables. If only some of the variables are specified in the FORM statement, canonical correlation analysis is used to determine the number of lags included in the state vector for the remaining variables not specified by the FORM statement. If the FORM statement includes specifications for all the variables listed in the VAR statement, the state vector is completely defined and the canonical correlation analysis is not performed.
In the introductory example shown in the preceding section, the F[2,3] parameter is not significant. (The parameters estimation output shown in Figure 18.6 gives the t statistic for F[2,3] as 0.06. F[3,3] and F[3,1] also have low significance with t < 2.)
The following statements reestimate this model with F[2,3] restricted to 0. The FORM statement is used to specify the state vector and thus bypass the canonical correlation analysis.
proc statespace data=in out=out lead=10; var x(1) y(1); id t; form x 2 y 1; restrict f(2,3)=0; run;
The final estimates produced by these statements are shown in Figure 18.9.

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