**Next message:**Edmond Offermann: "Re: [ROOT] fit."**Previous message:**Avi Malki: "[ROOT] fit."**In reply to:**Avi Malki: "[ROOT] fit."**Next in thread:**Edmond Offermann: "Re: [ROOT] fit."**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

In case of TGraph (where by definition no errors are defined), TGraph::Fit recompute the errors given the chisquare and NDF. If you print the errors on the parameters or SetOptFit, you will see the correct errors. Result of fit p0 = 0.644571 +- 0.016115 p1 = -1.03577 +- 0.181181 See your program modified below. Rene Brun { gStyle->SetOptFit(111); 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; 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*"); printf("Result of fit\n"); int npar = f1->GetNpar(); for (int i=0;i<npar;i++) { printf(" %s = %g +- %g\n",f1->GetParName(i),f1->GetParameter(i),f1->GetParError(i)); } } On Sun, 1 Aug 2004, Avi Malki wrote: > > 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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