[ROOT] strange error message when fitting

From: Sébastien Gadrat (gadrat@clermont.in2p3.fr)
Date: Thu May 13 2004 - 15:27:07 MEST


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



This archive was generated by hypermail 2b29 : Sun Jan 02 2005 - 05:50:08 MET