integral using TF1

From: Shamim, Mansoora <shamimm_at_phys.ksu.edu>
Date: Tue, 26 Apr 2005 16:24:28 -0500

Hi,
I am trying to fit a simple exponential function to my histogram. Please see the macro. Fit range is between 40, 55.
Total entries in my histogram are 1743. After fitting the function when I calculate the integral of teh function between 40 and 55 using TF1 it turns out to be 1821.71. Does anyone have any idea what I am doing wrong. Thanks for help
Mansoora

Double_t fitf(Double_t *x, Double_t *par)   {

     
      Double_t fitval = TMath::Exp(par[0]+x[0]*par[1]);
     
     return fitval;

  }
  void myfit()
  {

     TFile *f = new TFile("sumdata_all.root");   

     TCanvas *c1 = new TCanvas("c1","the fit canvas",500,400);   

     TH1F *hpx = (TH1F*)f->Get("allbmet");
     


  // Creates a Root function based on function fitf above

     TF1 *func = new TF1("fitf",fitf,40,55,2);
    


  // Sets initial values and parameter names

     // func->SetParameters(0,0);
     
     
     func->SetParNames("Constant","slope");
  


  // Fit histogram in range defined by function

     hpx->Fit(func,"r");
    
    


  // Gets integral of function between fit limits

     printf("Integral of function = %g\n",func->Integral(40,55));
     printf("Integral of function = %g\n",hpx->Integral(40,55));

  }

The output is the following

FCN=4.40737 FROM MIGRAD    STATUS=CONVERGED     490 CALLS         491 TOTAL
                     EDM=9.42813e-12    STRATEGY= 1      ERROR MATRIX ACCURATE
  EXT PARAMETER                                   STEP         FIRST
  NO.   NAME      VALUE            ERROR          SIZE      DERIVATIVE
   1  Constant     9.38380e+00   4.56946e-01   4.39444e-05   5.63492e-05
   2  slope       -9.83892e-02   1.01634e-02   9.77412e-07   2.15489e-03

Integral of function = 1821.71
Integral of histogram = 64 Received on Tue Apr 26 2005 - 23:24:52 MEST