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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