Hello Rooters, I cannot understand what ROOT does with fixed parameters whiel fitting: Here's a simple example, where h2 is a filled histogram { TF1 *fgaus=new TF1("fgaus","gaus",0,20); fgaus->SetParameters(1000,0.0,4); fgaus->SetParLimits(1,1,1); // To fix parameter 1 h2->Fit("fgaus","R"); } Here's ROOT output: Chi2 : 49349.3, FCN=288951 FROM MIGRAD STATUS=CONVERGED 77 CALLS 78 TOTAL EDM=0.00302017 STRATEGY= 1 ERROR MATRIX UNCERTAINTY 9.5 per cent EXT PARAMETER STEP FIRST NO. NAME VALUE ERROR SIZE DERIVATIVE 1 Constant 8.11320e+04 1.01253e+02 -0.00000e+00 1.07859e-05 2 Mean 3.83943e+00 fixed 3 Sigma 4.33514e+00 2.81893e-03 0.00000e+00 -2.45977e+02 Why is the value taken by ROOT different from the one I told it to use? If I change the statement fgaus->SetParLimits(1,1,1); by fgaus->SetParLimits(1,0,0); Root produces the output: Chi2 : 76219, FCN=54973.9 FROM MIGRAD STATUS=FAILED 224 CALLS 225 TOTAL EDM=0.0312136 STRATEGY= 1 ERR MATRIX APPROXIMATE EXT PARAMETER APPROXIMATE STEP FIRST NO. NAME VALUE ERROR SIZE DERIVATIVE 1 Constant 4.37930e+05 9.91646e+02 0.00000e+00 6.07322e-05 2 Mean -8.68125e+00 5.29279e-03 0.00000e+00 -4.31865e+01 3 Sigma 6.85899e+00 2.38016e-03 -0.00000e+00 2.60841e+03 Which means that dthe parameter is not fixed anymore. Am i doing somethig wrong? (I'm running Root 2.21.08 on Linux RedHat 6.0, egcs 1.1.2) ---------------------------- Mathieu de Naurois LPNHE Ecole Polytechnique 91128 Palaiseau Cedex denauroi@poly.in2p3.fr
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