Hi ROOTers, I have attached GIF file of that depicts two histograms. Top histogram is obtained by subtracting using TH1::Add(): h1->Add(h2,h3,1,-1); A glance at error bars of this histogram tells that errors weren't calculated correctly. The closer the value to zero the smaller is the error bar (it looks like that error=sqrt(abs(value))...) Bottom histogram contains right subtraction, done by handwritten function: void subtract(TH1* h1, TH1*h2, TH1* h3, double a=1, double b=1) { for (int i=0;i<h1->GetNbinsX(); i++) { double value = a*h1->GetBinContent(i)+b*h2->GetBinContent(i); double error = sqrt((a*a*h1->GetBinError(i)*h1->GetBinError(i))+(b*b*h2->GetBinError(i)*h2->GetBinError(i))); h3->SetBinContent(i,value); h3->SetBinError(i,error); } } Errors look right on the bottom one. Q: Is this a known feature and I just overlooked the documentation and there is a switch to calculate the errors right? If it is a bug, then I am amazed that: - it is overlooked - the same thing has been in early versions of PAW, is it a legacy thing... thanks, Dmitri Litvintsev /----------------------------------------------------------\ | Tel: (630) 840 5005 | | FAX: (630) 840 2968 | | office: 149-L CDF Trailers | | E-mail: litvinse@fnal.gov | \----------------------------------------------------------/
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