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RE: R MATRIX ALGORITHMICALLY SINGULAR

From: Grevel, Joachim <Joachim.Grevel>
Date: Thu, 10 Sep 2009 07:53:49 +0100

Hi Susan and Tianli,

I am not the mathematician to argue with the behaviour of matrices, but =
I have my way of "dealing" with this message.

First, it does not bother me too much. It comes (always?) with =
"Minimization successful" and that's better than having "rounding =
errors". It means that your minimum is fairly well described.

Second, not having the covariance step does not hinder me to develop my =
models further. You get the tables you ask for and with those you can =
create the scatter plots which tell you a lot about the problems of your =
model.

Third, I am a happy user of PsN which gives me the option to add =
"-retries=6 -picky" thus running the model six times with slightly =
altered starting values. Sometimes I find among the six results one =
where the covariance step was executed (it may not be the one with the =
lowest OFV). This way I can compare models and identify the parameter =
that might be either redundant or not supported by enough data.

Other NMusers will answer more competently,

Joachim


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-----Original Message-----
From: owner-nmusers
[mailto:owner-nmusers
Sent: 10 September 2009 04:01
To: Hudachek,Susan
Cc: nmusers
Subject: Re: [NMusers] R MATRIX ALGORITHMICALLY SINGULAR


Hi Susan,

The most common reason is that you got too many parameters. But if there =
is
someone who could summarize all other possible reasons for this kind of
error, it would be really appreciated.
If your model is not over-parameterized, there's one way to avoid it. =
You
could try adding "Matrix=S" into $COV block. This would give you a =
similar
estimate of covariance matrix if your sample size is large enough.
Hope it helps,

Tianli
****************************************************
Tianli Wang
PhD Candidate
Department of Pharmaceutics
University of Minnesota

On Sep 9 2009, Hudachek,Susan wrote:

> Greetings! I have run several models and the covariance steps have =
been
> unsuccessful due to the following error:
>
>R MATRIX ALGORITHMICALLY SINGULAR
>COVARIANCE MATRIX UNOBTAINABLE
>R MATRIX IS OUTPUT
>T MATRIX - EQUAL TO RS*R, WHERE S* IS THE INVERSE OF S - IS OUTPUT
>
> Does anyone have an idea as to what this indicates and how to 'fix" =
it?
> Thanks in advance for any help/input you can offer!
>Susan
>
>Susan Hudachek, M.S., Ph.D.
>Animal Cancer Center
>Veterinary Teaching Hospital
>Colorado State University
>300 West Drake Road
>Fort Collins, CO 80523-1620
>PHONE: (970) 219-7599
>FAX: (970) 297-1254
>EMAIL: Susan.Hudachek
>
Received on Thu Sep 10 2009 - 02:53:49 EDT

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