From: Guo Wei-Wei <wwguocn_at_gmail.com>

Date: Mon 04 Sep 2006 - 05:33:55 GMT

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Mon Sep 04 16:51:32 2006

Date: Mon 04 Sep 2006 - 05:33:55 GMT

Dear all,

I met a problem while doing SEM by sem package. I got a negative chi-square of null model. Because the theoretical value of chi-square cannot be negative, I checked the source code of sem.R in sem package and I found the Chi-square of null model was computed by the following expression:

result$chisqNull <- (N - 1) * (sum(diag(S %*% diag(1/diag(S)))) + log(prod(diag(S))))

I think the reason for negative Chi-square is the too small value of prod(diag(S)) of my data. I'm working on a data.frame named emc.data from a sample of a 16-item questioinnaire. The variance of items are

> diag(cov(emc.data))

** EMC1 EMC2 EMC3 EMC4 EMC5 EMC6 EMC7 EMC8
**
0.3622224 0.2350041 0.2488009 0.2901653 0.3195399 0.3107343 0.3436622 0.2345912

** EMC9 EMC10 EMC11 EMC12 EMC13 EMC14 EMC15 EMC16
**
0.2621680 0.3230400 0.4039245 0.3803105 0.2773370 0.4348342 0.2757216 0.3405252

The fit indices of RMSEA and GFI are good, so I think the problem might be solve by another way for computing the Chi-square of null model. I'm not well trained in maths, so I come for help. Any advise is appreciated.

Best wishes,

Wei-Wei

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Mon Sep 04 16:51:32 2006

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