rf109_chi2residpull.C

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/// \file
/// \ingroup tutorial_roofit
/// \notebook -js
/// Basic functionality: Calculating chi^2 from histograms and curves in RooPlots, making histogram of residual and pull
/// distributions
///
/// \macro_image
/// \macro_output
/// \macro_code
/// \author 07/2008 - Wouter Verkerke

#include "RooRealVar.h"
#include "RooDataSet.h"
#include "RooGaussian.h"
#include "RooConstVar.h"
#include "TCanvas.h"
#include "TAxis.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);
   gPad->SetLeftMargin(0.15);
   frame1->GetYaxis()->SetTitleOffset(1.6);
   frame1->Draw();
   c->cd(2);
   gPad->SetLeftMargin(0.15);
   frame2->GetYaxis()->SetTitleOffset(1.6);
   frame2->Draw();
   c->cd(3);
   gPad->SetLeftMargin(0.15);
   frame3->GetYaxis()->SetTitleOffset(1.6);
   frame3->Draw();
}