How Chi2 is calculated

Roofit and RooStats
1

Hi All,

I Have a question regarding how the chi2 of a fit is calculated in RooFit.

Here is my set-up:

  • model: two CBs and a Chebychev
  • fit: weighted least squares (minimum chi-square)
  • binned, weighted data supplied through a RooDataHist

The crux of the code is:

RooRealVar x("x","Mass", 1000.0, 5000.0);
...
 TFile f(fileName.c_str());
  TH1F *h = (TH1F*) f.Get(("eeMass_h_pt" + histName + "_y00_075_w").c_str());
  h->Sumw2(); //should already have been done
  h->Rebin(2);
  RooDataHist d("data","data",x,h);

  
  RooChi2Var chi2("chi2","chi2", tot, d,Extended(),DataError(RooAbsData::SumW2));
  RooMinuit minuit(chi2);
  minuit.migrad();
  minuit.minos();

Which works fine and converges successfully. The model (‘tot’ in code) has 10 free parameters and the fit is on a histogram with 40 bins = 30 degrees of freedom.

My issue is this:
When I do chi2.getVal() I get values like ~500
When I do xframe->chiSquare(10); I get values like ~2.8

Clearly, calculating the chi2/ndof from method 1 gives ~500/30 = 17, NOT 2.8.

I know exactly how the xframe->chiSquare(10) method works and from a visual inspection of the fit the value seems about right.

Can anyone tell me how chi2.getVal() works, and why it is so different to the xframe->chiSquare(10) method?