The IRT Procedure

where assignment represents: var-list < = parameter-spec>

The MEAN statement specifies the factor mean parameters in connection with the FACTOR statement.

In each assignment of the MEAN statement, you list the var-list that you want to specify for their means. Optionally, you can provide a list of parameter specifications in a parameter-spec after an equal sign for each var-list. The syntax of the MEAN statement is exactly the same as that of the VARIANCE statement. For more information, see the VARIANCE statement.

You can specify the following five types of the parameters for the means in the MEAN statement:

For example, consider a confirmatory factor model with latent factors F1, F2, F3, F4, F5. The following MEAN statement illustrates the five types of specifications in five assignments:

mean F1 ,
     F2 = (3.0),
     F3 = 1.5,
     F4 = fmean1,
     F5 = fmean2(0.6);

In this statement, the mean of F1 is specified as an unnamed free parameter. For this mean, PROC IRT generates a parameter name with the _Parm prefix appended with a unique integer (for example, _Parm1). The mean of F2 is an unnamed free parameter with an initial value of 3.0. PROC IRT also generates a parameter name for this mean. The mean of F3 is a fixed value of 1.5. This value stays the same during the estimation. The mean of F4 is a free parameter named fmean1. The mean of F5 is a free parameter named fmean2 with an initial value of 0.6.

If you do not use the MEAN statement, all the means of latent factors are fixed to 0 by default. For the confirmatory FACTOR models, you can override these default fixed 0 values by using the MEAN statement specifications. However, you cannot override the fixed 0 factor means for the exploratory FACTOR model.