Setting Covariance to Zero in Lavaan

Question

In lavaan in R, when using the sem() function, the covariance values are automatically populated. However, I want to force one of the covariance values to be zero while using the sem() function, i.e. without moving to the manual lavaan() function.

How can that be done?

Answer 1

See http://lavaan.ugent.be/tutorial/syntax2.html

I'm just copying from there, the following code is self-explanatory

# three-factor model
   visual =~ x1 + x2 + x3
  textual =~ x4 + x5 + x6
  speed   =~ NA*x7 + x8 + x9
# orthogonal factors
   visual ~~ 0*speed
  textual ~~ 0*speed
# fix variance of speed factor
    speed ~~ 1*speed

Note that the fist loadings on each latent factor are, by default, fixed to 1 - if you want to estimate them, you would use something like NA*x7 as above. And fixing a covariance to zero is done with something like visual ~~ 0*speed -- if you don't include this line, the covariances between latent variables are automatically included in the model.

The lavaan homepage is a very good source on lavaan.