Hi,
I just found out there seems to be a bug in ROOT when you
subtract histograms:
Instead of subtracting the values in the stats array (sumw,
sumwx), they are added resulting e.g. in a corrupt mean value
for the histogram.
I crosschecked the subtraction with the same histograms in PAW,
where I get the mean value I expected.
Just as an example:
Take a histogram of your choice, an subtract it with itself:
form1 = new TFormula("form1","abs(sin(x)/x)");
sqroot = new TF1("sqroot","x*gaus(0) + [3]*form1",0,10);
sqroot->SetParameters(10,4,1,20);
h1f = new TH1F("h1f","Test random numbers",200,0,10);
h1f->SetFillColor(45);
h1f->FillRandom("sqroot",10000);
h1f->Draw();
h1f->Add(h1f, -1.);
h1f->Draw();
Have a look at mean, rms,...
The ROOT version I use is:
3.01/06 29 August 2001
Cheers,
Marc-Andre.
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Marc-Andre Pleier Tel. : +0049 6221 516506
MPI Kernphysik Fax : +0049 6221 516603
Postfach 103980 Email: Marc-Andre.Pleier
D-69029 Heidelberg @mpi-hd.mpg.de
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