The IRT Procedure (Experimental)

The left panel of Figure 51.9 shows that the same set of four factors is used in both the rows and the columns. This yields six nonredundant covariances (variances are not included) to specify. In general, for a var-list list that has k variables in the COV statement, you can specify distinct covariance parameters. The variable order of the var-list list is important. For example, the left panel of Figure 51.9 corresponds to the following COV statement:

cov F1-F4 = phi1-phi6;

This statement is equivalent to the following statement:

cov F2 F1 = phi1,
    F3 F1 = phi2, F3 F2 = phi3,
    F4 F1 = phi4, F4 F2 = phi5, F4 F3 = phi6;

Another way to assign distinct parameter names that have the same prefix is to use the so-called prefix name. For example, the following COV statement is exactly the same as the preceding statement:

cov F1-F4 = 6*phi__; /* phi with two trailing underscores */

In the COV statement, phi_ _ is a prefix name that has the root phi. The notation 6* means that this prefix name is applied six times, resulting in a generation of the six parameter names phi1, phi2, …, phi6 for the six covariance parameters.

The root of the prefix name should have only a few characters so that the generated parameter name is not longer than 32 characters. To avoid unintentional equality constraints, the prefix names should not conflict with other parameter names.

You can also specify the within-list covariances as unnamed free parameters, as shown in the following statement:

cov F1-F4;

This statement is equivalent to the following statement:

cov F2 F1,
    F3 F1, F3 F2,
    F4 F1, F4 F2, F4 F3;

The right panel of Figure 51.9 illustrates the application of the between-list covariance specification. The set of row variables is different from the set of column variables. You intend to specify the cross covariances of the two sets of variables. There are four of these covariances in the figure. In general, for and variable names in the two variable lists (separated by an asterisk) in a COV statement, there are distinct covariances to specify. Again, variable order is very important. For example, the right panel of Figure 51.9 corresponds to the following between-list covariance specification:

cov F1 F2 * F3 F4 = phi1-phi4;

This is equivalent to the following statement:

cov  F1 F3 = phi1, F1 F4 = phi2,
     F2 F3 = phi3, F2 F4 = phi4;

You can also use the prefix name specification for the same specification, as shown in the following statement:

cov  F1 F2 * F3 F4 = 4*phi__ ; /* phi with two trailing underscores */

You can specify different types of parameters for the list of covariances. For example, you use a list of parameters that have mixed types in the following statement:

cov F1-F4 = phi1(0.1)  0.2  phi3  phi4(0.4) (0.5) phi6;

This statement is equivalent to the following statement:

cov F2 F1 = phi1(0.1) ,
    F3 F1 = 0.2       , F3 F2 = phi3,
    F4 F1 = phi4(0.4) , F4 F2 = (0.5), F4 F3 = phi6;

Notice that an initial value that follows a parameter name is associated with the free parameter. Therefore, in the original mixed list specification, 0.1 is interpreted as the initial value of the parameter phi1, but not as the initial estimate of the covariance between F3 and F1. Similarly, 0.4 is the initial value of the parameter phi4, but not the initial estimate of the covariance between F4 and F2.

However, if you indeed want to specify that phi1 is a free parameter without an initial value and 0.1 is an initial estimate of the covariance between F3 and F1 (while keeping all other things the same), you can use a null initial value specification for the parameter phi1, as shown in the following statement:

cov F1-F4 = phi1() (0.1)  phi3  phi4(0.4) (0.5) phi6;

This way, 0.1 becomes the initial estimate of the covariance between F3 and F1. Because a parameter list that has mixed types might be confusing, you can break down the specifications into separate assignments to remove ambiguities. For example, you can use the following equivalent statement:

cov F2 F1 = phi1 ,
    F3 F1 = (0.1)     , F3 F2 = phi3,
    F4 F1 = phi4(0.4) , F4 F2 = (0.5), F4 F3 = phi6;

If you provide fewer parameters than the number of covariances in the variable lists, all the remaining parameters are treated as unnamed free parameters. For example, the following statement assigns a fixed value to cov(F1,F3) while treating all the other three covariances as unnamed free parameters:

cov  F1 F2 * F3 F4 = 1.0;

This statement is equivalent to the following statement:

cov  F1 F3 = 1.0, F1 F4, F2 F3, F2 F4;

If you intend to fill up all values by the last parameter specification in the list, you can use the continuation syntax [...], [..], or [.], as in the following example:

cov  F1 F2 * F3 F4 = 1.0 phi [...];

This means that cov(F1,F3) is a fixed value of 1 and all the remaining three covariances are free parameters named phi. The last three covariances are thus constrained to be equal by having the same parameter name.

However, you must be careful not to provide too many parameters. For example, the following statement results in an error:

cov  F1 F2 * F3 F4 = 1.0 phi2(2.0) phi3 phi4 phi5 phi6;

The parameters after phi4 are excessive.

In exploratory analysis, all factor covariances are fixed at zero for the unrotated or orthogonally rotated solutions. For confirmatory analysis, by default all factor covariances are free parameters. You can also use the COV statement to override these default covariance parameters in situations where you want to set parameter constraints or provide initial or fixed values.