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The Model Building Language
The EQ Statement
The EQ statement is used to constrain any two free parameters to be equal. The syntax is:
EQ Statement1 AND Statement2
where the two statements identify one free parameter each. To impose constraints of
equality over more than two parameters the EQ statement is repeated the necessary
number of times with the same Statement1 and another Statement2.
It should be observed that the parentheses which are available to impose constraints of
equality in all statements generally is the most simple and convenient method to impose
constraints of equality. This method is not general, however, and there are many situations
where it is not possible to express the needed constraints with the parentheses. If, for
example, we need to express constraints of equality on the regressions of two different
manifest variables on two different latent variables this cannot be done with the parentheses. Or if, to take another example, we want to constrain a variance in a latent variable to
be equal to regression coefficient this cannot be done with the parenthesis method.
The EQ statement is general, however, and may be used to express any kind of equality
constraint. Suppose, for example, that we want to constrain a series of regressions
between six variables (y1-y6) in a simplex model to be equal. This may be done with the
following set of EQ statements:
EQ
EQ
EQ
EQ
REL
REL
REL
REL
y1
y1
y1
y1
->
->
->
->
y2
y2
y2
y2
AND
AND
AND
AND
REL
REL
REL
REL
y2
y3
y4
y5
->
->
->
->
y3
y4
y5
y6
An EQ statement to constrain the error variance of the manifest variable X1 to be equal
to the regression coefficient for the regression of X1 on x would be:
EQ VAR X1&
AND
REL x -> X1
Defining Scales
The SCL statement may be used to identify one variable as the scaling variable for a latent
variable. This thus implies that there will be a fixed relation of unity between these variables. The SCL command is issued when the SCL button is clicked. The syntax is:
SCL variable1 variable2
where variable2 is used to define the scale of variable1. For example, the command:
SCL g NUMSER
implies that the latent variable g will have its scale defined by the observed variable
NUMSER. The command
SCL Gv Vz
causes the latent variable Gv to have its scale defined by the latent variable Vz.
Limitations of the MB language
Even though the MB language allows a wide range of models to be formulated, not all
Structural Equation Modeling Made Simple