rf609_xychi2fit.C

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/// \file
/// \ingroup tutorial_roofit
/// \notebook -js
///  'LIKELIHOOD AND MINIMIZATION' RooFit tutorial macro #609
///
///  Setting up a chi^2 fit to an unbinned dataset with X,Y,err(Y)
///  values (and optionally err(X) values)
///
/// \macro_image
/// \macro_output
/// \macro_code
/// \author 07/2008 - Wouter Verkerke 


#include "RooRealVar.h"
#include "RooDataSet.h"
#include "RooPolyVar.h"
#include "RooConstVar.h"
#include "RooChi2Var.h"
#include "TCanvas.h"
#include "TAxis.h"
#include "RooPlot.h"
#include "TRandom.h"

using namespace RooFit ;


void rf609_xychi2fit()
{
   // C r e a t e   d a t a s e t   w i t h   X   a n d   Y   v a l u e s
   // -------------------------------------------------------------------

   // Make weighted XY dataset with asymmetric errors stored
   // The StoreError() argument is essential as it makes
   // the dataset store the error in addition to the values
   // of the observables. If errors on one or more observables
   // are asymmetric, one can store the asymmetric error
   // using the StoreAsymError() argument

   RooRealVar x("x","x",-11,11) ;
   RooRealVar y("y","y",-10,200) ;
   RooDataSet dxy("dxy","dxy",RooArgSet(x,y),StoreError(RooArgSet(x,y))) ;

   // Fill an example dataset with X,err(X),Y,err(Y) values
   for (int i=0 ; i<=10 ; i++) {

      // Set X value and error
      x = -10 + 2*i;
      x.setError( i<5 ? 0.5/1. : 1.0/1. ) ;
      
      // Set Y value and error 
      y = x.getVal() * x.getVal() + 4*fabs(gRandom->Gaus()) ;
      y.setError(sqrt(y.getVal())) ;

      dxy.add(RooArgSet(x,y)) ;    
   }



   // P e r f o r m   c h i 2   f i t   t o   X + / - d x   a n d   Y + / - d Y   v a l u e s
   // ---------------------------------------------------------------------------------------

   // Make fit function
   RooRealVar a("a","a",0.0,-10,10) ;
   RooRealVar b("b","b",0.0,-100,100) ;
   RooPolyVar f("f","f",x,RooArgList(b,a,RooConst(1))) ;

   // Plot dataset in X-Y interpretation
   RooPlot* frame = x.frame(Title("Chi^2 fit of function set of (X#pmdX,Y#pmdY) values")) ;
   dxy.plotOnXY(frame,YVar(y)) ;

   // Fit chi^2 using X and Y errors 
   f.chi2FitTo(dxy,YVar(y)) ;

   // Overlay fitted function
   f.plotOn(frame) ;
   
   // Alternative: fit chi^2 integrating f(x) over ranges defined by X errors, rather
   // than taking point at center of bin
   f.chi2FitTo(dxy,YVar(y),Integrate(kTRUE)) ;

   // Overlay alternate fit result
   f.plotOn(frame,LineStyle(kDashed),LineColor(kRed)) ;  


   // Draw the plot on a canvas
   new TCanvas("rf609_xychi2fit","rf609_xychi2fit",600,600) ;
   gPad->SetLeftMargin(0.15) ; frame->GetYaxis()->SetTitleOffset(1.4) ; frame->Draw() ;
   

}