How to retrieve a histogram from a fit in roofit

Roofit and RooStats
1

Hello,

I am trying to fit one histogram to another in roofit and I believe I have the fitting part down but now I cannot figure out how to retrieve the histogram that was allowed to vary. Here is the general idea of the process I am using:

[color=#00FF00]
RooRealVar x(“x”,“x”,-2.0,2.0);

RooDataHist data(“data”,“data”,x,hdata);
RooDataHist sig(“sig”,“sig”,x,hsig);

RooHistPdf sigPdf(“sigPdf”,“sigPdf”,x,sig);

RooRealVar Nsig(Nsig,Nsig,650000,0,5000000);

RooAddPdf sum(“sum”,“sum”,RooArgSet(sigPdf),RooArgSet(Nsig));

sum.fitTo(data,Extended());
[/color]

I need a histogram from sig in order to find the error in an integral after fitting with the histogram data.

2

Hi,

I am not sure exactly which histogram do you need. The fit will give you a best value of Nsig, the fit parameter

Best Regards

Lorenzo

3

Lorenzo

Sorry, I should have been a bit more clear. I need the histogram for sig so I can pull and display the error in each bin after the fit. I understand that I can get the integral from the fit in the range with Nsig.getVal(); and Nsig.getError(); but I want to be able to display the error in each bin from the fit.

thanks for helping,

George

4

Hi,

Would you like to get the error in the fitted function values after the fit in each bin ?
You can do this by using the option RooFit::VisualizeError(fitResult) in the plotOn method of the pdf .

See figure 1.3 of root.cern.ch/download/doc/roofi … t_3.00.pdf

Cheers

Lorenzo

5

Lorenzo,

Sorry, that didn’t seem to work. In that the function VisualizeError() doesn’t seem to be defined. Perhaps I can define what I want in another way. In Root for a normal TH1F* histogram I can use the function histogram->Integral(min,max); to get the error between min and max I can do this

[color=#00FF00]Double_t error = 0;
for(Int_t i=min; i<=max; i++){
Double_t binerror = histogram->GetBinError(i);
error=sqrt(binerror2+error2);

}

cout<<error<<endl;[/color]

so is there anyway to do that with the data types RooDataHist, RooHistPdf, or RooAddPdf?

Thank you for all your help!

George