What is the ProfileX BinError/StdError

ROOT
1

Hi,rooters,

Here is an example about the TProfile.
A 1d-histogram (TH1D *h1) is obtained by TProfileX of a TH2D *h2.
I want to know the binerror of the TH1D *h1, which I guess is equal to the std Dev(ProjectionY in each x bin) / sqrt(sum of the each x-axis bin counts). But the results shown in the figure doesn’t look that way.

The code is:

#include "TF2.h"
#include "TH2.h"
#include "TMath.h"
 
double g2(double *x, double *par) {
   double r1 = double((x[0]-par[1])/par[2]);
   double r2 = double((x[1]-par[3])/par[4]);
   return par[0]*TMath::Exp(-0.5*(r1*r1+r2*r2));
}
double fun2(double *x, double *par) {
   double *p1 = &par[0];
   double *p2 = &par[5];
   double *p3 = &par[10];
   double result = g2(x,p1) + g2(x,p2) + g2(x,p3);
   return result;
}
 
void example_profile() {
   const int npar = 15;
   double f2params[npar] =
      {100,-3,3,-3,3,160,0,0.8,0,0.9,40,4,0.7,4,0.7};
   TF2 *f2 = new TF2("f2",fun2,-10,10,-10,10, npar);
   f2->SetParameters(f2params);
 
   //Create an histogram and fill it randomly with f2
   TH2F *h2 = new TH2F("h2","from f2",10,-5,5,10,-5,5);
   int nentries = 2000;
   h2->FillRandom("f2",nentries);

   //================================ 
   TCanvas *c1 = new TCanvas("","",400,600);
	 c1->Divide(1,2);
	 c1->cd(1);
	 h2->Draw("colz");

	 c1->cd(2);
	 TH1D *h1 = h2->ProfileX();
	 h1->Draw();

 }

The results is:

Snipaste_2022-05-05_09-32-56
Snipaste_2022-05-05_09-32-561109×627 32 KB

binx = 7 ---- the std Dev = 1.992 (calculated by projectionY)
but the profilex results in the lower left figure is not equal to 1.992 or 1.992/sqrt(125).

Besides, I want to get the TH1D *h1 bin error by
h1->GetBinError(7),
But, the error message is:

Snipaste_2022-05-05_09-33-44
Snipaste_2022-05-05_09-33-441229×179 7.67 KB

Best regards,
hurricane…

2

h1 is not known outside the C++ script.

3

Hi,couet,

I got it. But one more question:
How to calculate the bin error of a TH1D obtained by TProfile?

4

@moneta will know better.
(In any case it is better to put the GetBinError call in the script itself)

5

Thank you, couet.
I have solved the problem by call the GetBinError in my script, the bin error of the profileX histogram is equal to sigma(i)/sqrt(ni), where i is the binx.

6

TH2::ProfileX returns a TProfile, not a TH1.

7

Thank you for pointing this. You’re right. The function called in my cript is right.