Re: [ROOT] Problems with ChiSquare

From: Rene Brun (Rene.Brun@cern.ch)
Date: Sat Feb 16 2002 - 10:45:16 MET 

Hi Johannes,

If the fit looks good but the chisquare wrong, it is symptomatic
of wrong errors for your histogram. In particular, when you fill
histograms with weights, you should call TH1::Sumw2 before filling,
otherwise ROOT will take bin_error = sqrt(bin_content).
I suggest to call Sumw2 for all your histograms involved in the fit.
If this does not solve this problem, please send me a small tar file
with your histogram in a file.root and a script. Make sure
that the script is autonomous such that I can use it directly.

Rene Brun

On Sat, 16 Feb 2002, Johannes wrote:

> Hi Rooters,
> 
> I have some problems with the calculation of the
> chisquare of a signal+background histogram fit.
> Although the fit both of the signal and the combined
> signal+background looks quite good, the chisquare is
> by far too high.
> I already tried to set the fit limits close by the
> signal gauss, but it doesn't help.
> 
> 
> Here's my code:
> 
> TF1* gaussian = new TF1 ("gaussian",
> "2*[0]*(1/(sqrt(2*pi)*[2]))*exp(-.5*((x-[1])/[2])^2)",0.,75000.);
> 
> TF1* ceby = new TF1("ceby",     
> "[0]*cheby_0+[1]*cheby_1+[2]*cheby_2+[3]*cheby_3+[4]*cheby_4");
> 
> TF1* fit_func = new TF1("fit_func",
> "(2*[0]*(1/(sqrt(2*pi)*[2]))*exp(-.5*((x-[1])/[2])^2))+[3]*cheby_0+[4]*cheby_1+[5]
>   *cheby_2+[6]*cheby_3+[7]*cheby_4",10000.,75000.);
> 
> 
> [...]
> gaussian->SetParameter(0,start_parameter[0]);
> gaussian->SetParameter(1,start_parameter[1]);
> gaussian->SetParameter(2,start_parameter[2]);
> ceby->SetParameter(3,start_parameter[3]);
> ceby->SetParameter(4,start_parameter[4]);
> ceby->SetParameter(5,start_parameter[5]);
> ceby->SetParameter(6,start_parameter[6]);
> ceby->SetParameter(7,start_parameter[7]);
>   
>       calibration_histos[i]->Fit("ceby","0+","");
>                   // 0(zero)=Store, do not draw
> TH1F* htemp=(TH1F*)calibration_histos[i]->Clone();
> htemp->SetName("htemp");
> htemp->Reset();
> background=calibration_histos[i]->GetFunction("ceby");
> for (int bin=min_bin;bin<max_bin;bin++)
> {
>    float x;
>    x=calibration_histos[i]->GetBinCenter(bin);
>    double value, difference;
>    value=ceby->Eval(x);
>   
> difference=fabs(value-calibration_histos[i]->GetBinContent(bin));
>    htemp->Fill(x,difference);
> }
> htemp->Fit("gaussian","0+","");
> signal=htemp->GetFunction("gaussian");
> 
> signal->GetParameters(&parameters[0]);
> background->GetParameters(&parameters[3]);
> fit_func->SetParameters(parameters);
>       calibration_histos[i]->Fit("fit_func","0+","");
>       for (int grumpf=0; grumpf<8;grumpf++)
>       {
>          cerr << endl << parameters[grumpf] << endl;
>          cerr << fit_func->GetParameter(grumpf);
>       }
> 
> calibration_histos[i]->Draw("");
> htemp->Draw("same");
> gaussian->Draw("same");
> fit_func->Draw("same");
> c1->Update();
> fit_parameters_act.chisqr=fit_func->GetChisquare();
> cerr<<endl<<"CHISQUAREGAUSS:"<<gaussian->GetChisquare()
> << endl;
> cerr<<endl<<"CHISQUAREFIT_FUNC"<<fit_parameters_act.chisqr
> << endl;
> 
> [...]
> 
> Hope i didn't overlook something completely trivial...
> 
> 
> Regards
> 
> 
> Johannes 
> 
> 
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