Hi Miro,
This is solved in 3.04/01
Rene Brun
On Wed, 11 Dec 2002, Miroslav Helbich wrote:
>
>
> Hi Rene, Fons and other rooters,
>
> I think this behaviour of the
> TGraphAsymmErrors::GetErrorY(int point) and TGraphAsymmErrors::GetErrorY(int point)
> is not as expected.
>
> root [6] gr1 = TGraphErrors(1)
> (class TGraphErrors)146016688
> root [7] gr1->SetPoint(0,1.,1.)
> root [8] gr1->SetPointError(0.,0.,0.5)
> root [11] gr1->GetErrorY(0)
> (const Double_t)5.00000000000000000e-01 <----- correct
>
> root [12] gr2 = new TGraphAsymmErrors(1)
> (class TGraphAsymmErrors*)0x8ebe288
> root [13] gr2->SetPoint(0,1.,1.)
> root [14] gr2->SetPointError(0,0.,0.,0.5,0.5)
> root [15] gr2->GetErrorY(0)
> (const Double_t)7.07106781186547573e-01 <------ sqrt(2)*0.5
>
> I would expect the errors to be the same since the graphs are the same.
>
> This formula is used to calculate the chi2 which then leads to a
> factor of 2 lower chi2 for the same data set defined with different class.
>
> I understand it is difficult to fit data with assymetric error bars
> and no simple prescription can be given.
>
> So far the error is calculated as
> return TMath::Sqrt(elow*elow + ehigh*ehigh);
>
> I would suggest something of the following
> (elow+ehigh)/2.
> sqrt((elow*elow+ehigh*ehigh)/2.)
> max(elow,ehigh)
>
> I am not sure which one is the best - it depends on the problem. Probably #3 or #1 would do.
>
> Miro
>
>
>
> Double_t TGraphAsymmErrors::GetErrorY(Int_t i) const
> {
> // This function is called by GraphFitChisquare.
> // It returns the quadratic error along Y at point i.
>
> if (i < 0 || i >= fNpoints) return -1;
> if (!fEYlow && !fEYhigh) return -1;
> Double_t elow=0, ehigh=0;
> if (fEYlow) elow = fEYlow[i];
> if (fEYhigh) ehigh = fEYhigh[i];
> return TMath::Sqrt(elow*elow + ehigh*ehigh);
> }
>
This archive was generated by hypermail 2b29 : Sat Jan 04 2003 - 23:51:23 MET