Re: [ROOT] Histogram fitting and errors

From: Rene Brun (Rene.Brun@cern.ch)
Date: Wed Jun 25 2003 - 18:38:10 MEST


Hi Ludyvine,

When you fill your histograms with weights, you should call
  myhist->Sumw2();
before filling the histograms, such that Root can compute/store the sum of
squares of weights for each bin.
Once you have called Sumw2(), the errors will be correctly computed
and automatically used by the Fit functions.

Rene Brun

Ludyvine Morand wrote:
> 
> Dear ROOTers,
> 
> I am doing histograms fitting with the Fit() method from TH1 class. As
> far as I understand the error used to do the fitting is the statistical
> error sqrt(N). True ?
> Actually my histograms were filled with events weighted (y=N*w), so I
> want the Fit() method to use another error than sqrt(y) that I believe
> it is using. Does anybody know how I should do this ?
> 
> Thanks !
> Ludyvine.
> 
> --
> Ludyvine Morand
> DAPNIA/SPhN - Bat 703 Piece 156 - Orme des Merisiers
> CEA Saclay - 91191 Gif-sur-Yvette cedex - FRANCE
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