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Mplus Discussion > Structural Equation Modeling >
 S.Arunachalam posted on Thursday, June 28, 2012 - 11:03 am
Hello Prof. Muthen

A clarification on usage of Model Test in multi-group analysis.I have two groups, and trying to check if the regression parameter with label a is equal to parameter b.

When I ran the code
Model test:
a = b
wald-chi sq test is non-significant
Wald Test of Parameter Constraints
Value 0.981
Degrees of Freedom 1
P-Value 0.3219

However if use
Model test:
a - b = 0

the results are
Wald Test of Parameter Constraints
Value 7.280
Degrees of Freedom 1
P-Value 0.0070

I thought in both ways am testing the same difference in parameter test. Please help
 Eric Deemer posted on Friday, July 18, 2014 - 1:08 pm
I have a question similar to the previous poster. I'm doing a multiple group analysis and I want to test the noninvariance of a particular path coefficient. Do I need to label the coefficient in the overall model command? My input is below:

MODEL: st BY svs1-svs8;
scientid by atmr sdcr sciid;
tapp BY newagq1 newagq4 newagq7;
sapp by newagq2 newagq5 newagq8;

tapp on scientid st;
sapp on scientid st;
scientid on st;

tapp ind scientid st;
sapp ind scientid st;

Model other:
scientid on st(a);
Model science:
scientid on st(b);

Model test:

 Bengt O. Muthen posted on Friday, July 18, 2014 - 4:13 pm
The way you label them is correct. Model test should have

0 = a-b;
 Eric Deemer posted on Friday, July 18, 2014 - 4:37 pm
Thanks so much, Bengt.

 Aidan posted on Monday, May 11, 2015 - 8:37 am
Dear Bengt,

I have run a multigroup latent growth model and am trying to compare the two groups for significant differences in the mean and variance of the two latent factors. Using the code below (I have also tested an alternative form of the MODEL TEST command, where p1=p21, and so on) produces a Wald test with a very low, significant, p-value for the omnibus test of all four parameters. However, when I examine each parameter one-by-one, none of them show a significant difference between groups.

Model Group 1:
[int] (p1);
[slope] (p2);
int (p3);
slope (p4);

Model Group 2:
[int] (p21);
[slope] (p22);
int (p23);
slope (p24);

Model test:
0 = p1-p21;
0 = p2-p22;
0 = p3-p23;
0 = p4-p24;

Can you tell me if I am doing anything wrong? It seems odd that none of the individual parameters are close to significance while the omnibus test is significant - I am not sure how one would report this in a useful fashion, since no specific difference between the two groups can be identified.

I have searched the forum archives for an answer, but haven't seen this described before - apologies if I have overlooked anything. Thank you.
 Bengt O. Muthen posted on Monday, May 11, 2015 - 6:15 pm
The joint test has more power to reject than each individual test so the outcome is quite possible. You can try to use a likelihood-ratio chi-2 difference test instead of the Wald test to check if you get the same result.
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