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...
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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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