rf109_chi2residpull.C: 'BASIC FUNCTIONALITY' RooFit tutorial macro #109

//////////////////////////////////////////////////////////////////////////
//
// 'BASIC FUNCTIONALITY' RooFit tutorial macro #109
// 
// Calculating chi^2 from histograms and curves in RooPlots, 
// making histogram of residual and pull distributions
//
//
//
// 07/2008 - Wouter Verkerke 
// 
/////////////////////////////////////////////////////////////////////////

#ifndef __CINT__
#include "RooGlobalFunc.h"
#endif
#include "RooRealVar.h"
#include "RooDataSet.h"
#include "RooGaussian.h"
#include "TCanvas.h"
#include "RooPlot.h"
#include "RooHist.h"
using namespace RooFit ;


void rf109_chi2residpull()
{

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

  // Create observables
  RooRealVar x("x","x",-10,10) ;

  // Create Gaussian
  RooRealVar sigma("sigma","sigma",3,0.1,10) ;
  RooRealVar mean("mean","mean",0,-10,10) ;
  RooGaussian gauss("gauss","gauss",x,RooConst(0),sigma) ;

  // Generate a sample of 1000 events with sigma=3
  RooDataSet* data = gauss.generate(x,10000) ;

  // Change sigma to 3.15
  sigma=3.15 ;


  // P l o t   d a t a   a n d   s l i g h t l y   d i s t o r t e d   m o d e l
  // ---------------------------------------------------------------------------

  // Overlay projection of gauss with sigma=3.15 on data with sigma=3.0
  RooPlot* frame1 = x.frame(Title("Data with distorted Gaussian pdf"),Bins(40)) ;
  data->plotOn(frame1,DataError(RooAbsData::SumW2)) ;
  gauss.plotOn(frame1) ;


  // C a l c u l a t e   c h i ^ 2 
  // ------------------------------

  // Show the chi^2 of the curve w.r.t. the histogram
  // If multiple curves or datasets live in the frame you can specify
  // the name of the relevant curve and/or dataset in chiSquare()
  cout << "chi^2 = " << frame1->chiSquare() << endl ;


  // S h o w   r e s i d u a l   a n d   p u l l   d i s t s
  // -------------------------------------------------------

  // Construct a histogram with the residuals of the data w.r.t. the curve
  RooHist* hresid = frame1->residHist() ;

  // Construct a histogram with the pulls of the data w.r.t the curve
  RooHist* hpull = frame1->pullHist() ;

  // Create a new frame to draw the residual distribution and add the distribution to the frame
  RooPlot* frame2 = x.frame(Title("Residual Distribution")) ;
  frame2->addPlotable(hresid,"P") ;

  // Create a new frame to draw the pull distribution and add the distribution to the frame
  RooPlot* frame3 = x.frame(Title("Pull Distribution")) ;
  frame3->addPlotable(hpull,"P") ;



  TCanvas* c = new TCanvas("rf109_chi2residpull","rf109_chi2residpull",900,300) ;
  c->Divide(3) ;
  c->cd(1) ; frame1->Draw() ;
  c->cd(2) ; frame2->Draw() ;
  c->cd(3) ; frame3->Draw() ;
  
}