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st: SEM & degrees of freedom


From   Daniel Coulombe <d_coulombe@sympatico.ca>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: SEM & degrees of freedom
Date   Fri, 20 Dec 2013 15:39:21 -0500

Hello,

Can anybody tell me why the number of degrees of freedom is the same for the
two following model specifications :

1: sem (Verbal@bv -> word similar) (NVerbal@bn -> matrices seq_quan)
(Spatial@bs -> pattern r_design), covstruct(_lexogenous, diagonal) stand
cov( NVerbal*Verbal Spatial*Verbal Spatial*NVerbal)

2: sem (Verbal -> word similar) (NVerbal -> matrices seq_quan) (Spatial ->
pattern r_design), covstruct(_lexogenous, diagonal) stand cov(
NVerbal*Verbal Spatial*Verbal Spatial*NVerbal)

In model 1, I constrain to equality the coefficients for each factors, thus
gaining 3 degrees of freedom over the unconstrained model 1. But when I run
these, I get 6 degrees of freedom for BOTH models!

LR test of model vs. saturated: chi2(6)   =     29.28, Prob > chi2 = 0.0001
for model 1		DF should be equal to 9
LR test of model vs. saturated: chi2(6)   =      9.76, Prob > chi2 = 0.1351 
for model 2		DF is correct (at 6)

Thanks for any help!

Daniel Coulombe


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