Adding histograms changes number of entries in ways I don't understand, please help

[Frigorifico]

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_ROOT Version: 6.19
_Platform: OS 10.14.3

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I include all the files and code at the end.

I have 2 files of “raw histograms” of measurements taken under different conditions, but the amount of data points is exactly the same.

I have also some “noise histograms” which is the same for both.

I add the raw histograms and subtract the noise like this:

    hHist.Add(hNoise,-1.0*n)

where n is just a scale factor. If I took 20 rounds of measurements for the noise but 15 for the actual measurements then the noise gets normalized and multiplied by 15 to match the scale of the measurements.

Everything was working fine when I noticed that one of them had way better results, I thought it was just the measurement conditions until I noticed the number of entries, one has easily 10 times more than the other.

It was my understanding that if I have 2 histograms with 10 entries and you add them up you get one with 20, so I would expect both of them to have the same number of entries. However this is just the case sometimes.

To investigate this I decided to print the number of entries after each step in the process, this is what I got (“sumando” means adding up in spanish):

First data set
sumando
3647.0
7294.0
10941.0
14588.0
18235.0
21882.0
25529.0
29176.0
32823.0
32823.0
noise entries
32823.0
data entries
32823.0
after noise subtraction
290765.26725
Second data set
sumando
3647.0
7294.0
10941.0
14588.0
18235.0
21882.0
25529.0
29176.0
32823.0
32823.0
noise entries
32823.0
data entries
32823.0
after noise subtraction
1301369.43743

As you can see after the noise is subtracted the amount of entries changes drastically, it even becomes fractionary. I do not understand this at all. I have been reading the documentation, but I don’t see anything about this.

What am i doing wrong?

testtest 2.zip (1.5 MB)

[Wile_E_Coyote]

Subtracting multiple histogram with same TH2F

I guess you could be interested in the TH1:Integral / TH2::Integral / TH3::Integral and / or the TH1::IntegralAndError / TH2::IntegralAndError / TH3::IntegralAndError methods. When you “subtract” histograms, the “number of entries” is recalculated in a different way than when you “add” them so, you may get “unexpected” / “weird” values. { TH1F *h1 = new TH1F("h1", "h1", 100, -5., 5.); h1->Sumw2(kTRUE); #if 0 /* 0 or 1 */ h1->FillRandom("gaus"); #else /* 0 or 1 */ for (Int_t i = 0; i < …

Subtracting histograms does not yield difference in entries

When I subtract two histograms, the number of entries in the subtracted histogram is not equal to the difference in the number of entries between the two histograms. It seems that the resulting histogram does not contain the contents of the underflow and overflow bins. This is not, however, the case for addition, which seems to preserve the sums just fine. Is this behavior expected? It is very unintuitive. Step-by-step demonstration: Get TFile and TTree and draw a branch into histograms (wit…

[Frigorifico]

thanks a lot, but I have a follow up question, in fact, Newbie made the same question but nobody answered it: am I supposed then to set the entries to what I know should be the correct value?, or should I leave it as it is?