Re: [ROOT] strange error message when fitting

From: Rene Brun (Rene.Brun@cern.ch)
Date: Thu May 13 2004 - 21:47:38 MEST


Hi Sebastien,

As explained by Eddy, it looks like your parameters are correlated.
Could you send a small file containing your histogram, plus
a short test macro?

Rene Brun

On Thu, 
13 May 2004, Sébastien Gadrat wrote:

> Hi Rooters,
> 
> I get a strange error message when fitting some mass plots. Among 
> information returned by Minuit after the completion of the fit, I get 
> "ERR MATRIX NOT POS-DEF" ! I have read in Minuit documentation the need 
> for a positive defined matrix around the minimum for a "physical" 
> function. I use the same method to fit five different mass spectra 
> (differents cuts are used to generate each of these mass spectra) and 
> only one fit returned this error message. So I think the parametrization 
> is good. According to Minuit documentation, it could also be due to a 
> numerical inaccuracies or a non-physical region. What can I do to 
> distinguish between these two possibilities and to get a positived 
> defined matrix ?
> 
> Here is the output I get for this fit (the fit function is made of three 
> exponentials):
> 
> FCN=153.796 FROM MIGRAD    STATUS=CONVERGED      91 CALLS          92 TOTAL
>                     EDM=2.47125e-08    STRATEGY= 1      ERR MATRIX NOT 
> POS-DEF
>  EXT PARAMETER                APPROXIMATE        STEP         FIRST
>  NO.   NAME      VALUE            ERROR          SIZE      DERIVATIVE
>   1  const0       5.72969e+03   7.36832e+01   4.07529e-01  -1.22906e-06
>   2  const1      -8.46984e-01   7.65250e-03   3.44875e-06   7.87522e-02
>   3  const3       9.15351e-02   7.21665e-03   1.82483e-05  -4.06709e-02
>   4  slope1       2.19718e+00   6.64370e-03   2.93137e-06  -4.12211e-02
>   5  slope2       2.08281e+00   6.22503e-03   2.84950e-06   4.62693e-02
>   6  slope3       5.18135e-01   1.17508e-02   3.77755e-05   7.06570e-03
> 
> Thanks in advance,
> Regards,
> 
> Sebastien
> 



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