Modifying an MSTRUCT Model from a Reference Model

In this section, it is assumed that you use a REFMODEL statement within the scope of a MODEL statement and the reference model (or base model) is also an MSTRUCT model. The reference model will be referred to as the old model, while the model that makes reference to the old model is referred to as the new model. If the new model is not intended to be an exact copy of the old model, you can use the following extended MSTRUCT modeling language to make modifications within the scope of the MODEL statement for the new model. The syntax is similar to, but not exactly the same as, the ordinary MSTRUCT modeling language, as in the section MSTRUCT Statement. The syntax for respecifying or modifying an MSTRUCT model takes the following form:

MSTRUCT ;

MATRIX _COV_ parameters-in-matrix ;

MATRIX _MEAN_ parameters-in-matrix ;

First, in the respecification, you should not put any VAR= list in the MSTRUCT statement, as you would do for specifying the original base model. The reason is that parameter respecifications in the new model make reference to the variables in the VAR= list of the old model. Therefore, the VAR= list in the new model is implicitly assumed to be exactly the same as that in the old model. This renders the specification of a VAR= list of the MSTRUCT statement of the new model unnecessary. Because the VAR= option is the only possible option in the MSTRUCT statement, it also implies that the entire MSTRUCT statement is only optional for the new model.

Second, you can use the MATRIX _COV_ and MATRIX _MEAN_ statements to modify from the old model by using the same syntax as in ordinary MSTRUCT modeling language. In addition, in the respecification syntax, you can use the missing value '.' to drop a parameter location from the old model.

The new model is formed by integrating with the old model in the following ways:

Duplication:

If you do not specify in the new model a parameter location that exists in the old model, the old parameter specification is duplicated in the new model.

Addition:

If you specify in the new model a parameter location that does not exist in the old model, the new parameter specification is used in the new model.

Deletion:

If you specify in the new model a parameter location that also exists in the old model and the new parameter is denoted by the missing value '.', the old parameter specification is not copied into the new model.

Replacement:

If you specify in the new model a parameter location that also exists in the old model and the new parameter is not denoted by the missing value '.', the new parameter specification replaces the old one in the new model.

For example, consider the following statements for a two-group analysis:

   proc tcalis;
      group 1 / data=d1;
      group 2 / data=d2;
      model 1 / group=1;
         mstruct var=V1-V6;
         matrix _COV_ [1,1] = 6*vparm (8.),
                      [2,]  = cv21,
                      [3,]  = cv31,
                      [4,]  = cv41 cv42 cv43,
                      [5,]  = cv51 cv52 cv53 cv54,
                      [6,]  = cv61 cv62 cv63 cv64 cv65;
      model 2 / group=2;
         refmodel 1;
         matrix _COV_ [2,]  = 3.,
                      [3,2] = cv32,
                      [4,]  = . . . ,
                      [5,]  = . . . ,
                      [6,]  = . . . ;
      run;

In these statements, you specify model 2 by referring to model 1 in the REFMODEL statement. Hence, model 2 is called the new model that refers to the old model, model 1. Because they are not respecified in the new model, all parameters on the diagonal of the covariance matrix are duplicated from the old model for the new model. Similarly, parameter locations associated with the cv54, cv64, and cv65 parameters are also duplicated in the new model.

An added parameter in the new model is cv32 for the covariance between V3 and V2. This parameter location is not specified in the old model.

In the new model, parameters for the covariances between the variable sets {V1 V2 V3} and {V4 V5 V6} are all deleted from the old model. The corresponding parameter locations for these covariances are given missing values '.' in the new model, indicating that they are no longer free parameters as in the old model. Deleting these parameters amounts to setting the corresponding covariances to fixed zeros in the new model.

Finally, covariance between V2 and V1 is changed from a free parameter cv21 in the old model to a fixed constant in the new model. This illustrates the replacement rule of the respecification syntax.