Hi Rene, While trying to do linear fit on a graph using root 4.00/08 and red-hat 7.3 I noticed I am getting large errors for the fit parameters. I found a reference to a similar problem (see below) when trying the same code as below, I got the same results of like Voropaev S.G., 1 p0 6.44571e-01 8.45154e-01 2.18445e-04 -1.48898e-14 2 p1 -1.03577e+00 9.50208e+00 2.45598e-03 -8.82907e-16 Was this problem indeed fixed ?, or am I doing something wrong ? ================================================================= The e-mail { Voropaev S.G. wrote: > > Hello, > I have new problem with fitting unsorted data. I am trying to fit unsorted > array X,Y to line. I use object TGraph. Parameters of line is well, but error > is very strange. > Test macro with only 5 data point. (In real life number of data point is > larger, but problem still exist): > > Result of fitting: p0 = 0.64 +- 0.85 !, p1 = -1.04 +- 9.5 !!! > > Where is error? It is my error or not? > > I use ROOT 1.02/00 1 august 1997 ( Valery Fine version) on Windows NT > > With best regards Voropaev S.G. This problem is now fixed in the development version 1.03/02. Result of fitting: p0 = 0.644 +- 0.023, p1 = -1.036 +- 0.256 The fitting algorithm in version 1.02 from TGraph and TGraphErrors objects assumed errors = 1 ! I have now implemented an algorithm that really uses the error specified in TGraphErrors and compute an approximation of the errors for TGraph objects. Thanks for reporting Rene Brun */ float Xarr[5]; float Yarr[5]; Xarr[0] = 0.062893; Yarr[0] = 0.584000; Xarr[1] = 0.000000; Yarr[1] = 0.646000; Xarr[2] = 0.062893; Yarr[2] = 0.572000; Xarr[3] = 0.125786; Yarr[3] = 0.497000; Xarr[4] = 0.125786; Yarr[4] = 0.533000; // Xarr[5] = 0.000000; Yarr[5] = 0.626000; // Xarr[6] = 0.125786; Yarr[6] = 0.529000; // Xarr[7] = 0.251572; Yarr[7] = 0.432000; // Xarr[8] = 0.125786; Yarr[8] = 0.534000; // Xarr[9] = 0.251572; Yarr[9] = 0.441000; // Xarr[10] = 0.188679; Yarr[10] = 0.480000; // Xarr[11] = 0.000000; Yarr[11] = 0.627000; // Xarr[12] = 0.377358; Yarr[12] = 0.338000; // Xarr[13] = 0.188679; Yarr[13] = 0.467000; // Xarr[14] = 0.251572; Yarr[14] = 0.405000; // Xarr[15] = 0.000000; Yarr[15] = 0.646000; // Xarr[16] = 0.503145; Yarr[16] = 0.252000; // Xarr[17] = 0.251572; Yarr[17] = 0.366000; c1 = new TCanvas("c1","proba"); TF1 *f1 = new TF1("f1","pol1"); f1->SetRange(0,0.15); TGraph *tg = new TGraph(5, Xarr, Yarr); tg->Fit("f1","R","AP"); tg->Draw("A*"); } ------------------------------------------------------------------- Avi Malki Department of Nuclear Physics School of Physics and Astronomy Tel-Aviv University Israel University - Phone : 972-3-6405280 Fax : 972-3-6406369 Home - Phone : 972-3-5081734 E-Mail: malki@tauphy.tau.ac.il WWW : http://www-nuclear.tau.ac.il/~malki/ -------------------------------------------------------------------
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