Re: [ROOT] Errors used in TGraphErrors

From: Edmond Offermann (
Date: Sun Aug 01 2004 - 21:28:18 MEST

Hi Stilianos,

Your 2 expressions are identical ?

Anyhow, your question is answered at
In particular ,

"The chisquare is computed as the sum of the quantity below at each

                     (y - f(x))**2
         ey**2 + ((f(x+exhigh) - f(x-exlow))/2)**2

 where x and y are the point coordinates.

 In case the function lies below (above) the data point, ey is ey_low

  thanks to Andy Haas ( for adding the case with
            University of Washington

 a little different approach to approximating the uncertainty in y
because of the
 errors in x, is to make it equal the error in x times the slope of the
 The improvement, compared to the first method (f(x+ exhigh) -
 is of (error of x)**2 order. This approach is called "effective
variance method".
 This improvement has been made in version 4.00/08 by Anna Kreshuk.:


--- Stilianos Kesisoglou <> wrote:

> Hi,
> I was wondering, which is the method implemented for the assignment
> of
> errors in
> the TGraphErrors class? The above class accepts errors on both x and
> y, so
> how
> exaclty the errors are combined for the fit? I assume there is a
> chi^2
> function like:
>              [ y-f(x) ]**2                     [ y-f(x) ]
>            ------------------       or        ( -------------- ) **2
>                  s**2                               s
> What is the s in the above expressions? 
> Thanks!
> Stelios.

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