[ROOT] Fitting a Histogram

From: Alexander Dietz (Alexander.Dietz@mpi-hd.mpg.de)
Date: Mon Jul 08 2002 - 12:54:38 MEST


Hello Rooters,

I want to understand how the fitting is done in ROOT. When I am fitting
data to a model I use the Maximum Likelihood fitting method (i.e.
'H1FitLikelihood' in TH1.cc).

1. What is the mathematical background of this fitting method? Can
somebody give me the formula(s)? And uses it Gaussian or Poisson
statistics? And if yes, is it possible to change it?

2. It should be possible to specify a user-defined fitting function using
'TVirtualFitter::Fitter(myhist)->SetFCN(MyFittingFunction)'. But how must
this function made of? How must it work? What is the meaning of the
parameters in 'extern void MyFittingFunction(Int_t &npar, Double_t *gin,
Double_t &f, Double_t *u, Int_t flag);'?

Thanks much in advance for helping me,

   A. Dietz



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