Problem in Fitting Status in Roofit

Hi experts,
I am trying to fit exponential function with my dataset(using RooDecay) . The fit Status Converges but the NOT POSDEF . I dont find any idea to fix this. The error is shown below.


EIGENVALUES OF SECOND-DERIVATIVE MATRIX:
-1.1216e-03 7.8769e-01 1.0749e+00 2.1385e+00
MINUIT WARNING IN HESSE
============== MATRIX FORCED POS-DEF BY ADDING 0.003260 TO DIAGONAL.
FCN=538269 FROM HESSE STATUS=NOT POSDEF 23 CALLS 156 TOTAL
EDM=1.77229e-05 STRATEGY= 1 ERR MATRIX NOT POS-DEF
EXT PARAMETER APPROXIMATE INTERNAL INTERNAL
NO. NAME VALUE ERROR STEP SIZE VALUE
1 dt 8.92747e-02 1.56443e-02 2.60157e-05 -1.38154e+00
2 mean 3.77404e-01 6.60677e-02 4.49178e-05 -1.17977e+00
3 sigma 2.74317e-01 4.83087e-02 2.93141e-04 -2.67330e+00
4 t 1.53458e+00 2.63378e-03 2.99858e-04 5.47741e-02
ERR DEF= 0.5
EXTERNAL ERROR MATRIX. NDIM= 25 NPAR= 4 ERR DEF=0.5
2.448e-04 -1.031e-03 7.471e-04 -5.937e-07
-1.031e-03 4.367e-03 -3.170e-03 -1.911e-07
7.471e-04 -3.170e-03 2.343e-03 1.180e-06
-5.937e-07 -1.911e-07 1.180e-06 6.937e-06
ERR MATRIX NOT POS-DEF
PARAMETER CORRELATION COEFFICIENTS
NO. GLOBAL 1 2 3 4
1 0.99703 1.000 -0.997 0.987 -0.014
2 0.99796 -0.997 1.000 -0.991 -0.001
3 0.99113 0.987 -0.991 1.000 0.009
4 0.19942 -0.014 -0.001 0.009 1.000
ERR MATRIX NOT POS-DEF

Can any body help me please?
Thank you
Cp

Hi,

the program checked if the error matrix is positively defined and this is not the case. There is no general solution to this as this behaviour is usually a symptom of a fragile fit. Perhaps is worth revising the model and its parametrisation.

Danilo

Hi Danilo,
Thanks for your response . I just want to know if my fit Status is NOT POSDEF (But Converge) and I am getting a good fit . Then , Is that a correct plot? And another thing I am not getting anything like COVARIANCE MATRIX CALCULATED SUCCESSFULLY. So, Is that due to parametrisation ?

Thank You
Cp

Hi,

as it was said it depends on the fit.
Did you change anything?

D

Hi Danilo,
Some times It shows Fit Status OK ,Sometime not. Now changing some Variable it shows ok.

Thank you
Cp