[R] appropriate covariance matrix for multiple nominal exogenous and multiple continuous endogenous variables in SEM

[Gus Jespersen]

Hi,
I would like to use the sem package to perform a path analysis (no 
latent variables) with a mixture of 2 nominal exogenous, 1 continuous 
exogenous, and 4 continuous endogenous variables.  I seek advice as to 
how to calculate the appropriate covariance matrix for use with the sem 
package. 

I have read through the polycor package, and am confused as to the use 
of "numeric" for the hetcor function.  Is this used synonymous with a 
continuous variable, or perhaps a nominal variable?

Thanks,
Gus