Error in structural equation model - "The model hasnegativedegrees of freedom"

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch 
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Sunil W
Sent: Sunday, December 04, 2005 11:03 AM
To: R-help at stat.math.ethz.ch
Subject: [R] Error in structural equation model - "The model 
hasnegativedegrees of freedom"

Hi John
Thanks a lot for the reply.

Could you suggest how I can correct this problem? I tried using a 
correlation matrix instead of raw moments, but still got the same 
error. I also fixed parameters v1,v2,v3,a1 at 1; then it gave me the 
error that the system is exatly singular.

To answer the points that you raised:

1. x1-x6 are not causes; they are just indicatiors. Does that change 
my model?

2. Is the size of the dataset going to be a real limitation? 
How can I take care of that?

-----------------------------------
Dear Sunil,

There are 7*8/2 = 28 raw moments among the 7 observed variables. Of 
these,
6*7/2 = 21 are used for the moments among the 6 fixed-exogenous 
variables, leaving 28 - 21 = 7 df. You model has 11 free parameters. 
So df for the model = 11 - 7 = -4.

Some additional comments:

If you're using raw moments, why isn't there a constant variable in 
the model?

Do you really intend x1 -- x6 to be causes, rather than indicators, of 
m1 and m2?

Why are there no normalizing constraints on the latent variables?

Do you really want to fit a model like this to so small a data set?

I hope this helps,
John

John Fox
Department of Sociology
McMaster University
Hamilton, Ontario
Canada L8S 4M4
905-525-9140x23604
http://socserv.mcmaster.ca/jfox

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch 
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Sunil W
Sent: Wednesday, November 30, 2005 10:08 PM
To: r-help at stat.math.ethz.ch
Subject: [R] Error in structural equation model - "The model has 
negativedegrees of freedom"

Hi

I am running a structural equation model with R using the

sem command;

am getting the following error:

"Error in sem.default : The model has negative degrees of

freedom = -4"

My model is as follows:

s_model = specify.model()
x1->m1, b1,NA
x2->m1, b2,NA
x3->m2, b3,NA
x4->m2, b4,NA
x5->m2, b5,NA
x6->m2, b6,NA
m1->y, a1,NA
m2->y, a2,NA
m1<->m1, v1,NA
m2<->m2, v2,NA
y<->y, v3,NA


x1-x6 are observed independent variables, m1 and m2 are the latent 
variables and y is the observed dependent variable. I use the 
raw.moments command for calculating the covariance matrix,

based on a

data with 147 observations.

The command that I use is as follows:

s = sem(s_model,S=R,obs.variables=colnames(R),
fixed.x=c('x1','x2','x3','x4','x5','x6'), raw=TRUE)


I would appreciate any help on this; I am new to structural equation 
models and realize that I may be making a silly error.