Dear Folks, I have already posted this stuff a fews ago in ROOTTalk but I still need to understand a few things. I also read the manual in TMinuit. I am doing the fitting and access the fitted parameters and errors using two fitting option: (a) TH1D::Fit("f_name"); (b) TH1D::Fit("f_name","E"); As a cross check I dumped a few parameter values and their errors. -----------a-a-a-a-a-a-a-a-a-a-a-a-a-a------------------------------------------ This is the result for (a) TH1D::Fit("f_name"); LINE FIT(Data) Slope: 0.00162291 +/- 0.000605578 <====== P0 Int. : 0.644645 +/- 0.440121 <====== P1 FCN=274.613 FROM MIGRAD STATUS=CONVERGED 1185 CALLS 1186 TOTAL EDM=7.07449e-07 STRATEGY= 1 ERROR MATRIX UNCERTAINTY 3.1 per cent EXT PARAMETER STEP FIRST NO. NAME VALUE ERROR SIZE DERIVATIVE 1 p0 6.44645e-01 4.40121e-01 -2.79232e-05 1.24430e-04 2 p1 1.62291e-03 6.05578e-04 -1.92359e-08 2.48881e-01 ------------------------------------------------------------------------------- --------b-b-b-b-b-b-b-b-b-b-b-b-b-b------------------------------------------- This is the result for (b) TH1D::Fit("f_name","E"); LINE FIT(Data) Slope: 0.00162291 +/- 0.000605869 <===== P0 Int. : 0.644645 +/- 0.445796 <===== P2 FCN=274.613 FROM MINOS STATUS=SUCCESSFUL 6620 CALLS 8331 TOTAL EDM=4.59478e-07 STRATEGY= 1 ERROR MATRIX ACCURATE EXT PARAMETER PARABOLIC MINOS ERRORS NO. NAME VALUE ERROR NEGATIVE POSITIVE 1 p0 6.44645e-01 4.45830e-01 -4.46699e-01 4.44894e-01 2 p1 1.62291e-03 6.05907e-04 -6.05686e-04 6.06052e-04 -------------------------------------------------------------------------------- In (a) the errors are reflected there in the summary. In (b) the errors do not exactly match. I mean 0.000605869(for P1) is almost the same with 4.45830e-01 (if qouting the PARABOLIC), but not equal. As a cross check, it is good to know that the errors you dumped are near to the MINOS algorithm, but this could be confusing. I have 3 questions: (1) Since the TMinuit members are public, then I expect to have same values with the one printed in the summary below. Am I Correct? (2) Why is the dumped values not reflected in (b)? Or Is the method: f->GetParError(nth_param) not enough? In that case the user will still add few more lines(i.e., TMinuit * minuit = new TMinuit ......; minuit->GetWhatIwant() ....)...I wanted to Know. (3) How do we qoute the errors in (b) when the dumped values are not reflected in the print out summary? The reason I use option (b) is because this gives a more reliable result. Thank you very much. Zaldy A. Nawang
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