RE: R MATRIX ALGORITHMICALLY SINGULAR
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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Quoted reply history
-----Original Message-----
From: [email protected]
[mailto:[email protected]]on Behalf Of [email protected]
Sent: 10 September 2009 04:01
To: Hudachek,Susan
Cc: [email protected]
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: [email protected]
>