I’ve just stumbled on something quite weird.
I tried to fit a few points on a line with a -well- a line.
More in detail: I draw a TGraph using 6 points (1,1), (2,2), (3,3), … (6,6). Then I fitted that using a TF1.
This is the result:
Why is the chi-squared so far from 1…?
It’s totally messed up. o.O
You are having a perfect fit and you have not provide any error (uncertainty on the data points) so it is expected that your chi2 value is very close to zero.
That makes sense but… Well, I definitely need to study this chapter again, because I remember that the reduced chi^2 should be about 1 for “best” fits. Isn’t it?
How does ROOT calculate it?
Isn’t it just: https://s3-eu-west-1.amazonaws.com/tutor2u-media/subjects/geography/studynoteimages/chi-squared-equation.png?mtime=20151023210421
where the “observed frequencies” are the y-values of the points in the input file, while the “expected frequencies” are the y-values evaluated from the fit function at the same x coordinates?
If I do the same calculation, I get different results than those given by ROOT.
This topic was automatically closed 14 days after the last reply. New replies are no longer allowed.