R help - Dec 2005 - Error in structural equation model - "The model has negative degrees 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.

Thanks
Sunil

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.
> 
> Thanks
> Sunil
> 
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