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. ----------------------------------------------------------------------------- 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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