Fit integral issue

Newbie
1

Attached is the test macro I am trying to calculate integral under the fit curve. I do not understand why the fit integral is approx (or maybe exactly) 10 times the expected value or the corresponding th1::integral value. Can some please explain?

void test() {
   
   TH1F* trial = new TH1F("","",100,-5, 5);
   TRandom3 rndgen;
   for (int i = 0; i < 10000; ++i)
   trial->Fill(rndgen.Gaus(0.,1.));
   trial->Scale(1./trial->GetEntries());
   TF1 *g = new TF1("g","gaus",-5.,5.);
   trial->Fit("g","R");
//   g->SetParameters(2,0,1);
   //default gaus integration method uses 6 points
   //not suitable to integrate on a large domain
   double r1 = g->Integral(-5.,5.);
   double r2 = trial->Integral(trial->FindBin(-5),trial->FindBin(5));
//   trial->GetYaxis()->SetRangeUser(0,2);
   trial->GetXaxis()->SetRangeUser(-5,5);
   gStyle->SetOptFit(1);
   trial->Draw();
 
   printf("g->Integral(-5,5)               = %g\n",r1);
   printf("hist->Integral(-5,5)            = %g\n",r2);

}

Result I get is

g->Integral(-5,5) = 0.0991864
hist->Integral(-5,5) = 1

ROOT Version: 6.26.14

2

Use instead:

double r2 = trial->Integral(trial->FindBin(-5),trial->FindBin(5), "width");
3

This worked. The values are much closer now, and probably will keep on getting close the better the fit. Can you explain what the “width” option did here?

Thank You

4

See ROOT: TH1 Class Reference