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TH1D filled by FillRandom is different from the one filled by GetRandom

Roofit and RooStats
#1

I have defined a function, “fun”. Based on this function, a histogram is filled by

        h1_func->FillRandom("fun",1000000);

While, if a histogram is filled by the random number of this function like this,

         for(int i=0;i<1000000;i++){
              h1_sample->Fill(func->GetRandom());
         }

the histogram h1_sample is different from h1_func.
Does any one know how it happens? Thanks in advance.
BTW, my code is based on ROOT Version: 6.16/00.

#2

Hello,

It looks like it is the implementation of FillRandom doing something different:

but @moneta can better comment on what.

#3

Hi,

I think you should get the same results, within statistical lfluctuations, if you set the number of function bins equal to the histogram ones:
func->SetNpx( h1_func->GetNbinsX())

In case of different bins, especially of the function bins are less than the histogram one you could get a significative different result. The default number of function bins is 100 .

Lorenzo

#4

Thank you very much.
I have tried to set the number of points to a very large value, but the result is the same.
I also generated a seriese of random numbers based on this function using an analytical way. This time the obtained histogram is consistent with the one filled by func->GetRandom(). My code is like this,test.cc (2.4 KB)

And the comparison of these three histograms is shown in the attached figure.comp.pdf (57.7 KB)

#5

Hi,

You need compatible bins between the TF1 and TH1 to have same result. In addition, TF1::GetRandom and TH1::FillRandom generate only in the range, while your analytical solution no. So there is a normalisation issue.
I have modified your program and now the three methods show compatible results

Lorenzo
test.cc (2.5 KB)

#6

Hi, Moneta
Thank you so much. It works with the modified program.
I was wrong when using the TH1 FillRandom method before.
FillRandom(“func”,Nsample) only generates randoms numbers within the range of TH1, not TF1.
Thanks again. This problem is solved.