Re: [ROOT] Errors

From: Ilia Ravinovich (ravini@wicc.weizmann.ac.il)
Date: Wed Aug 15 2001 - 14:24:01 MEST


On Wed, 15 Aug 2001, Rene Brun wrote:

Thanks Rene.

Ilia

> Ilia,
> You should call Sumw2 for the resulting histogram before making
> the operation if you want to have the correct error bars computed.
> See documentation of the TH1::Add functions.
> If your operand histograms already have Sumw2, this is not necessary.
> Example:
> {
>    gROOT->Reset();
>    TH1F *h1 = new TH1F("h1","h1",100,-3,3);
>    Int_t bin = h1->Fill(0,1911);
>    TH1F *h2 = new TH1F("h2","h2",100,-3,3);
>    h2->Fill(0,1537.57);
>    TH1F *h3 = h1->Clone("h3");
>    h3->Sumw2();
>    h3->Add(h1,h2,1,-1);
>    printf("Bin conte= %g\n",h3->GetBinContent(bin));
>    printf("Bin error= %g\n",h3->GetBinError(bin));
> }
> 
> Rene Brun
> 
> On Wed, 15 Aug 2001, Ilia Ravinovich wrote:
> 
> > Hi Rene,
> > 
> > In my macro I'm doing:
> > 
> > inv_reb_signal_n->Add(inv_reb_real_n,inv_reb_mixed_norm_n,1.,-1.);
> > inv_reb_signal_n->Print("all");
> > 
> > I checked the output for example bin #10:
> > 
> > fSumw[10]=373.426, x=1.145, error=43.7226
> > 
> > But the error is wrong because:
> > 
> > fSumw[10]=1911, x=1.145 (this is from 'inv_reb_real_n')
> > fSumw[10]=1537.57, x=1.145 (this is from 'inv_reb_mixed_norm')
> > 
> > 1911-1537.57=373.43, error=sqrt(1911+1537.57)=58.72
> > 
> > Am I doing something wrong?
> > 
> > Thanks,
> > 
> > Ilia
> > 
> > ************************************************************************
> > *                                                                      *
> > *   Ilia Ravinovich                                                    *
> > *   Department of Particle Physics             PPE Division            *
> > *   Weizmann Institute of Science              Bld 892, 1D-015         *
> > *   Rehovot 76100                              CERN, 1211 Geneva       *
> > *   ISRAEL                                     Switzerland             *
> > *                                                                      *
> > *   PHONE : 972 8 9343187                      41 22 7676437           *
> > *   FAX   : 972 8 9466253                      41 22 7677910           *
> > *   E-mail: Ilia.Ravinovich@weizmann.ac.il     Ilia.Ravinovich@cern.ch *
> > *                                                                      *
> > ************************************************************************
> > 
> 
> 

************************************************************************
*                                                                      *
*   Ilia Ravinovich                                                    *
*   Department of Particle Physics             PPE Division            *
*   Weizmann Institute of Science              Bld 892, 1D-015         *
*   Rehovot 76100                              CERN, 1211 Geneva       *
*   ISRAEL                                     Switzerland             *
*                                                                      *
*   PHONE : 972 8 9343187                      41 22 7676437           *
*   FAX   : 972 8 9466253                      41 22 7677910           *
*   E-mail: Ilia.Ravinovich@weizmann.ac.il     Ilia.Ravinovich@cern.ch *
*                                                                      *
************************************************************************



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