doc/v606/rf602__chi2fit_8C.html

#ifndef __CINT__

#include "RooGlobalFunc.h"

#endif

#include "RooRealVar.h"

#include "RooDataSet.h"

#include "RooGaussian.h"

#include "RooConstVar.h"

#include "RooChebychev.h"

#include "RooAddPdf.h"

#include "RooChi2Var.h"

#include "RooMinuit.h"

#include "TCanvas.h"

#include "TAxis.h"

#include "RooPlot.h"

using namespace RooFit ;


void rf602_chi2fit()

{


  // S e t u p   m o d e l

  // ---------------------


  // Declare observable x

  RooRealVar x("x","x",0,10) ;


  // Create two Gaussian PDFs g1(x,mean1,sigma) anf g2(x,mean2,sigma) and their parameters

  RooRealVar mean("mean","mean of gaussians",5) ;

  RooRealVar sigma1("sigma1","width of gaussians",0.5) ;

  RooRealVar sigma2("sigma2","width of gaussians",1) ;


  RooGaussian sig1("sig1","Signal component 1",x,mean,sigma1) ;

  RooGaussian sig2("sig2","Signal component 2",x,mean,sigma2) ;


  // Build Chebychev polynomial p.d.f.

  RooRealVar a0("a0","a0",0.5,0.,1.) ;

  RooRealVar a1("a1","a1",-0.2,0.,1.) ;

  RooChebychev bkg("bkg","Background",x,RooArgSet(a0,a1)) ;


  // Sum the signal components into a composite signal p.d.f.

  RooRealVar sig1frac("sig1frac","fraction of component 1 in signal",0.8,0.,1.) ;

  RooAddPdf sig("sig","Signal",RooArgList(sig1,sig2),sig1frac) ;


  // Sum the composite signal and background

  RooRealVar bkgfrac("bkgfrac","fraction of background",0.5,0.,1.) ;

  RooAddPdf  model("model","g1+g2+a",RooArgList(bkg,sig),bkgfrac) ;


  // C r e a t e   b i n n e d   d a t a s e t

  // -----------------------------------------


  RooDataSet* d = model.generate(x,10000) ;

  RooDataHist* dh = d->binnedClone() ;


  // Construct a chi^2 of the data and the model.

  // When a p.d.f. is used in a chi^2 fit, the probability density scaled

  // by the number of events in the dataset to obtain the fit function

  // If model is an extended p.d.f, the expected number events is used

  // instead of the observed number of events.

  model.chi2FitTo(*dh) ;


  // NB: It is also possible to fit a RooAbsReal function to a RooDataHist

  // using chi2FitTo().


  // Note that entries with zero bins are _not_ allowed

  // for a proper chi^2 calculation and will give error

  // messages

  RooDataSet* dsmall = (RooDataSet*) d->reduce(EventRange(1,100)) ;

  RooDataHist* dhsmall = dsmall->binnedClone() ;

  RooChi2Var chi2_lowstat("chi2_lowstat","chi2",model,*dhsmall) ;

  cout << chi2_lowstat.getVal() << endl ;


}