Hi Cozmin,
likely you want to use:
TProfile::Approximate(kTRUE);
to get a more useful estimate of the errors in pathological
cases.
For detailed info see:
http://root.cern.ch/root/html/TProfile.html#TProfile:GetBinError
Cheers
Otto
On Thu, 25 Nov 2004, Timis CN Dr (Physics) wrote:
> 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();
> }
>
>
>
--
Dr. Otto Schaile Department f"ur Physik, LMU M"unchen
Phone: +49 89 289 14070 Am Coulombwall 1, D-85748 Garching, Germany
Mobil: +49 160 1553598
FAX: +49 89 289 14072 EMail: Otto.Schaile@Physik.Uni-Muenchen.DE
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