Dear Rooters, is there anybody who can comment on the following effect? Just copy the following example into your ROOT-session: 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(); So, what's happening is that you subtract two identical histograms from each other and get - as expected - an empty histogram as result BUT the mean and rms are still the same of the original histogram being subtracted from itself... I think, what happens is the following: If you subtract two histograms, their values for sumw (stats[0]) and sumwx (stats[2]) are added for the resulting histogram instead of being subtracted. This will thus result in a wrong mean value. The way ROOT behaves right now, you can get two identical (non-empty!) histograms having different MEAN and RMS. I tried the same in PAW - there everything looks fine, and the above example would result in an empty histogram with mean = rms = 0. I tried this for ROOT-versions 3.01/06 29 August 2001 and 2.24/05 25 May 2000. If there's a good reason for this, I'd really like to know. Cheers, Marc-Andre P.S.: I've posted this already some time ago, but got no answer. I still think this is a severe problem... ----------------------------------------------------------------------------- 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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