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Hi Stilianos, Your 2 expressions are identical ? Anyhow, your question is answered at http://root.cern.ch/root/htmldoc/TGraph.html#TGraph:Fit In particular , "The chisquare is computed as the sum of the quantity below at each point: (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 (ey_high). thanks to Andy Haas (haas@yahoo.com) for adding the case with TGraphasymmerrors 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 line. The improvement, compared to the first method (f(x+ exhigh) - f(x-exlow))/2 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.: Eddy --- Stilianos Kesisoglou <kesisogl@fnal.gov> 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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