[ROOT] subtacting histograms, errrors (fwd)

From: Dmitri Litvintsev (litvinse@fnal.gov)
Date: Thu Jul 25 2002 - 21:35:00 MEST 

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

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