[ROOT] TProfile error for bins with one event

From: Timis CN Dr (Physics) (C.Timis@surrey.ac.uk)
Date: Thu Nov 25 2004 - 11:48:34 MET 

G'day  rooters,
 
I'm concerned about TProfile treatments of the bins with just one event. According to the TProfile source the default computation of errors is:
 
"     "  SQRT(Y)/SQRT(N) for Spread.eq.0,N.gt.0
 
In my case it seems like the error for the 1 event bins is set to 0 which might create problems for the subsequent fits.
I attached a small program ( similar to the one given as  example) that shows a similar problem. It seems that in hprof the one event bins are ignored. In hprof1 they are present and the error associated is zero.  The same thing occurs when I use the menu ("righth click" "ProfileX") to produce a ProfileX from the hprof_bidim 2D histogram.  When fitted with a  pol2 the two profile histograms (hprof, and hprof1) give completely different results.
 
I'm using root 4.01/02 on an RedHat Linux release 7.3 (Valhalla) kernel 2.4.20-18.7
 
Is there an option to remove the 1 event channels , or compute a reasonable error ?
 
Cheers,
Cozmin 
 
____________________________________________________________________ 
C. N. Timis
Department of Physics | Email: C.Timis@surrey.ac.uk 
University of Surrey  | tel: +44 (0)1483 68 69 05 
Guildford GU2 7XH     | fax: +44 (0)1483 68 67 81  
UK                    | 
_____________________________________________________________
 
{
 TCanvas *c1 = new TCanvas("c1","Profile histogram example",200,10,700,500);
  hprof  = new TProfile("hprof","Profile of pz versus px",100,-4,4,0,20);
  hprof1  = new TProfile("hprof1","Profile of pz versus px",100,-4,4,0,20);
 TH2F *hprof_bidim = new TH2F("hprof_bidim","",100,-4,4,100,0,20);
  Float_t px, py, pz;
  for ( Int_t i=0; i<1000; i++) {
     gRandom->Rannor(px,py);
     pz = px*px + py*py;
     hprof->Fill(px,pz,1);
     hprof_bidim->Fill(px,pz);
  }
hprof->Draw();
hprof1=hprof_bidim->ProfileX(); 
TCanvas *my = new TCanvas;
hprof_bidim->Draw();
TCanvas *my2 = new TCanvas;
hprof1->Draw();
}

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