Hi Alexander, Your errors along X seem to be far too large compared to your errors along Y. If I reduce the errors by a factor 5 or 10, I obtain a good fit. See below Rene Brun void prog(void) { float x[10]; float y[10]; float ex[10]; float ey[10]; Double_t alpha = .1; x[0]= 7508.84 ;ex[0]=alpha*0.037; x[1]= 1680.41 ;ex[1]=alpha*0.022 ; x[2]= 2093.11 ;ex[2]=alpha*0.05 ; x[3]= 2259.89 ;ex[3]=alpha*0.157; x[4]= 2474.87 ;ex[4]=alpha*0.065; x[5]= 2569.45 ;ex[5]=alpha*0.473; x[6]= 3100.43 ;ex[6]=alpha*0.272; x[7]= 4574.82 ;ex[7]=alpha*0.147; x[8]= 6041.46 ;ex[8]=alpha*0.15 ; y[0]=2614.53 ;ey[0]=0.013; y[1]=583.191 ;ey[1]=0.002; y[2]=727.33 ;ey[2]=0.009; y[3]=785.37 ;ey[3]=0.08; y[4]=860.564 ;ey[4]=0.005; y[5]=893.40 ;ey[5]=0.005; y[6]=1078.62 ;ey[6]=0.1; y[7]=1592.54 ;ey[7]=0.013; y[8]=2103.53 ;ey[8]=0.013; TCanvas* c1=new TCanvas("c1","c1",10,10, 400,300); TGraphErrors* graph=new TGraphErrors(9,x,y,ex,ey); graph->Draw("A*"); graph->Fit("pol2",""); graph->Draw(""); } FCN=819.438 FROM MIGRAD STATUS=CONVERGED 147 CALLS 148 TOTAL EDM=2.63628e-13 STRATEGY= 1 ERROR MATRIX UNCERTAINTY 6.4 per cent EXT PARAMETER STEP FIRST NO. NAME VALUE ERROR SIZE DERIVATIVE 1 p0 -3.67015e+00 1.67026e-02 1.19699e-08 -4.33296e-05 2 p1 3.49409e-01 1.13286e-05 -6.75892e-12 5.63994e-03 3 p2 -9.86409e-08 1.29352e-09 5.42316e-16 -3.08442e+02 On Mon, 30 Sep 2002, Alexander Dietz wrote: > Hello Rooters, > > I have a problem with doing a fit with a TGraphErrors. In the program > source (at the end of this mail) I want to perform a quadratic fit with > 'TGraphErrors", using the errors as specified. > When I use the fitting option 'W' (all errors set to 1) then the fit works > very good, giving a sensefull result. > But without the option 'W' the result of the fit is very, very bad and > wrong. > The correct result is, that parameter 1 is about 0.35. > But with the option 'W' this parameter is around 10^7 !!! > > So what can I do to get a correct fit which is using the errors I have > specified? > > > Regards > > Alexander Dietz > > > void prog(void) > { > float x[10]; > float y[10]; > float ex[10]; > float ey[10]; > > x[0]= 7508.84 ;ex[0]=0.037; > x[1]= 1680.41 ;ex[1]=0.022 ; > x[2]= 2093.11 ;ex[2]=0.05 ; > x[3]= 2259.89 ;ex[3]=0.157; > x[4]= 2474.87 ;ex[4]=0.065; > x[5]= 2569.45 ;ex[5]=0.473; > x[6]= 3100.43 ;ex[6]=0.272; > x[7]= 4574.82 ;ex[7]=0.147; > x[8]= 6041.46 ;ex[8]=0.15 ; > > y[0]=2614.53 ;ey[0]=0.013; > y[1]=583.191 ;ey[1]=0.002; > y[2]=727.33 ;ey[2]=0.009; > y[3]=785.37 ;ey[3]=0.08; > y[4]=860.564 ;ey[4]=0.005; > y[5]=893.40 ;ey[5]=0.005; > y[6]=1078.62 ;ey[6]=0.1; > y[7]=1592.54 ;ey[7]=0.013; > y[8]=2103.53 ;ey[8]=0.013; > > > TCanvas* c1=new TCanvas("c1","c1",10,10, 400,300); > TGraphErrors* graph=new TGraphErrors(9,x,y,ex,ey); > > graph->Draw("A*"); > graph->Fit("pol2","LEM"); > graph->Draw(""); > > double p0=graph->GetFunction("pol2")->GetParameter(0); > double p1=graph->GetFunction("pol2")->GetParameter(1); > double p2=graph->GetFunction("pol2")->GetParameter(2); > } > >
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