question about equality constraints

[Mark S.]

Hi All,

I am trying to modify a loading for one of my parameters and I can't quite figure out how to do it from reading the lavaan instructions.  

Here's what the instructions say about modifiers:

Modifiers

We have seen the use of the pre-multiplication mechanism (using the * operator) a number of times: to fix a parameter, to provide a starting value, and to label a parameter. We refer to these operations as modifiers, because they modify some properties of certain model parameters. More modifiers will be introduced later. 

Each term on the right hand side in a formula can have one modifier only. If you must specify more modifiers for the same parameter, you need to list the term multiple times in the same formula. For example:

f =~ y1 + y2 + myLabel*y3 + start(0.5)*y3 + y4

if I want the loading on one of my variables to be equal to negative one times whatever loading the analysis gives it, what do I do?  If, for example, i wanted y2 in the equation above to have -1* whatever loading is calculated by the program, I want to put "-1*y2" or "y2*-1" (which is what you can do in MPlus), but I don't know how to do that in lavaan.  

Can someone help?

Thanks!

Mark S. 

[Jeremy Miles]

Flipping the sign on a loading will have implications for other parameters. You can use a starting value and either make that negative, or give it the negative of the final value  that you know you will end up with.

f = ~start(-0.786)*y2

You can also do it by adding a non-linear constraint:

f = ~nonposvalue*y2

# constraints

nonposvalue < 0

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[Alex Schoemann]

I'll add that the Mplus code you provided is changing the starting values (the first approach Jeremy mentioned), it is not actually multiplying the parameter by -1.