To get random values from poisson distribution

Roofit and RooStats
1

Hello,

I will be very thankful to get some help for my code:
I have a Gaussian histogram of 4 bins. the code to get the histogram is below:

TH1F *h1 = new TH1F("h1","d=1.2 mm,t=0.4mm",8,0,24);
TF1 f1("f1", "TMath::Abs(1/(TMath::Sqrt(2)*TMath::Sqrt(TMath::Pi())*(1.06)))*TMath::Exp(-((x-6.6)*(x-6.6))/2.2472)", 0, 24);
 h1->FillRandom("f1",400);
 h1->Draw();

For each bin, I have error given by Poisson distribution. Therefore, I want to plot the same Gaussian histogram having value for each bin randomly selected by the Poisson distribution
But when I do it, the get random function is not selecting all the random values from poisson distribution.

[code]// Define poisson for different bins:

TF1 *f1 = new TF1(“f1”, “TMath::Poisson(x,1)”,0,6);
TF1 *f2 = new TF1(“f2”, “TMath::Poisson(x,120)”,70,180);
TF1 *f3 = new TF1(“f3”, “TMath::Poisson(x,272)”,200,360);
TF1 *f4 = new TF1(“f4”, “TMath::Poisson(x,7)”,0,30);

// To redefine the same histogram with randomly selected values

TH1F *h = new TH1F(“h”, “d=0.8 and t = 0.7mm”, 8, 0, 24);
for (Int_t i=0; i<1000; i++){ h->Fill(1.5, f1->GetRandom()); h->Fill(4.5, f2->GetRandom()); h->Fill(7.5, f3->GetRandom()); h->Fill(10.5, f4->GetRandom()); cout<GetMean()<<endl;}[/code]

Thank you.

Kajal

2

Hi,

To get Random number from the Poisosn distribution, you should use the TRandom::Poisson function and not create a TF1. The number you obtained with the TF1 are also generated according to the given TF1 range

Best Regards

Lorenzo

3

Hello Lorenzo,

Thank you for your help. It makes my code easier :smiley:
Also, I figured out that I had to include the TH1F in the “For loop” along with “delete h” to generate random numbers.

Thank you so much. :slight_smile:

Kajal