Dear root and statistics experts,
I tried to analyze the code behind the function GetConfidenceIntervals in TFitter.cxx .

I fitted data in TGraphErrors with a function like [0]+[1]Log10(X) and I’d like to get the associated confidence intervals for a 0.95 cl. So I used the function GetConfidenceIntervals and I got the result.
OK…my question (probably trivial!) is the following: why in the code of the file TFitter.cxx the result is multiplied (and not divided?!) by a factor TMath::Sqrt(f->GetChisquare())?

Thank you very much


When the Chisquare/ndf is > 1, it means you are under-estimate your fit error, so the confidence interval is enlarged to take this into account. The opposite happens when Chisquare/ndf is < 1.


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