How to access chi-squared/NDF after ->chi2FitTo()?

Roofit and RooStats
1

Hi experts,

I have a RooAddPDF and want to fit it to a RooDataHist. Currently, I use the default maximum likelihood fit by doing the following:

some_rooaddpdf.fitTo(some_roodatahist, Range(fitting_lowerbound,fitting_higherbound));

Now I want to do the same, but with a chi-squared fit instead. I guess I would have to use ->chi2FitTo() analogous to before, like this:

some_rooaddpdf.chi2FitTo(some_roodatahist, Range(fitting_lowerbound,fitting_higherbound));

Two questions:

  1. I am a bit confused by the documentation of chi2FitTo(). Could someone elaborate on the option ‘Extended’?

  2. How do I access the goodness-of-fit value, which is ‘chi-squared/NDF’? I know there is a tutorial showing how to calculate it when using a frame, but is there no way of simply accessing it when the chi-square fit has just been done? Also, the documentation of TFitResult does not show how to access it.

Any ideas? Thanks in advance!

2

Hi,

Let me comment about the Extended option. This is a standard way to perform fits, in this context the Chi2 equivalent of the Extended Likelihood fit. You find a lot of documentation about this, let me just link this one by Luis Lyons. In a nutshell it’s a way in which you can fit the normalisation of your pdf.

It should be easy for you to derive the reduced chi2, but I add in the loop @moneta and @jonas in case they have suggestions.

Best,
D