rf611_weightedfits.C

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/// \file
/// \ingroup tutorial_roofit
/// \notebook -js
/// Likelihood and minimization: Parameter uncertainties for weighted unbinned ML fits
///
/// ## Parameter uncertainties for weighted unbinned ML fits
///
/// Based on example from https://arxiv.org/abs/1911.01303
///
/// This example compares different approaches to determining parameter uncertainties in weighted unbinned maximum likelihood fits.
/// Performing a weighted unbinned maximum likelihood fits can be useful to account for acceptance effects and to statistically subtract background events using the sPlot formalism.
/// It is however well known that the inverse Hessian matrix does not yield parameter uncertainties with correct coverage in the presence of event weights.
/// Three approaches to the determination of parameter uncertainties are compared in this example:
///
/// 1. Using the inverse weighted Hessian matrix [`SumW2Error(false)`]
///
/// 2. Using the expression [`SumW2Error(true)`]
///    \f[
///      V_{ij} = H_{ik}^{-1} C_{kl} H_{lj}^{-1}
///    \f]
///    where H is the weighted Hessian matrix and C is the Hessian matrix with squared weights
///
/// 3. The asymptotically correct approach (for details please see https://arxiv.org/abs/1911.01303) [`Asymptotic(true)`]
///    \f[
///      V_{ij} = H_{ik}^{-1} D_{kl} H_{lj}^{-1}
///    \f]
///    where H is the weighted Hessian matrix and D is given by
///    \f[
///      D_{kl} = \sum_{e=1}^{N} w_e^2 \frac{\partial \log(P)}{\partial \lambda_k}\frac{\partial \log(P)}{\partial \lambda_l}
///    \f]
///    with the event weight \f$w_e\f$.
///
/// The example performs the fit of a second order polynomial in the angle cos(theta) [-1,1] to a weighted data set.
/// The polynomial is given by
/// \f[
///   P = \frac{ 1 + c_0 \cdot \cos(\theta) + c_1 \cdot \cos(\theta) \cdot \cos(\theta) }{\mathrm{Norm}}
/// \f]
/// The two coefficients \f$ c_0 \f$ and \f$ c_1 \f$ and their uncertainties are to be determined in the fit.
///
/// The per-event weight is used to correct for an acceptance effect, two different acceptance models can be studied:
/// - `acceptancemodel==1`: eff = \f$ 0.3 + 0.7 \cdot \cos(\theta) \cdot \cos(\theta) \f$
/// - `acceptancemodel==2`: eff = \f$ 1.0 - 0.7 \cdot \cos(\theta) \cdot \cos(\theta) \f$
/// The data is generated to be flat before the acceptance effect.
///
/// The performance of the different approaches to determine parameter uncertainties is compared using the pull distributions from a large number of pseudoexperiments.
/// The pull is defined as \f$ (\lambda_i - \lambda_{gen})/\sigma(\lambda_i) \f$, where \f$ \lambda_i \f$ is the fitted parameter and \f$ \sigma(\lambda_i) \f$ its
/// uncertainty for pseudoexperiment number i.
/// If the fit is unbiased and the parameter uncertainties are estimated correctly, the pull distribution should be a Gaussian centered around zero with a width of one.
///
/// \macro_image
/// \macro_code
/// \macro_output
///
/// \date November 2019
/// \author Christoph Langenbruch
 
#include "TH1D.h"
#include "TCanvas.h"
#include "TROOT.h"
#include "TStyle.h"
#include "TRandom3.h"
#include "TLegend.h"
#include "RooRealVar.h"
#include "RooFitResult.h"
#include "RooDataSet.h"
#include "RooPolynomial.h"
 
using namespace RooFit;
 
 
int rf611_weightedfits(int acceptancemodel=2) {
  // I n i t i a l i s a t i o n   a n d   S e t u p
  //------------------------------------------------
 
  //plotting options
  gStyle->SetPaintTextFormat(".1f");
  gStyle->SetEndErrorSize(6.0);
  gStyle->SetTitleSize(0.05, "XY");
  gStyle->SetLabelSize(0.05, "XY");
  gStyle->SetTitleOffset(0.9, "XY");
  gStyle->SetTextSize(0.05);
  gStyle->SetPadLeftMargin(0.125);
  gStyle->SetPadBottomMargin(0.125);
  gStyle->SetPadTopMargin(0.075);
  gStyle->SetPadRightMargin(0.075);
  gStyle->SetMarkerStyle(20);
  gStyle->SetMarkerSize(1.0);
  gStyle->SetHistLineWidth(2.0);
  gStyle->SetHistLineColor(1);
 
  //initialise TRandom3
  TRandom3* rnd = new TRandom3();
  rnd->SetSeed(191101303);
 
  //accepted events and events weighted to account for the acceptance
  TH1D* haccepted = new TH1D("haccepted", "Generated events;cos(#theta);#events", 40, -1.0, 1.0);
  TH1D* hweighted = new TH1D("hweighted", "Generated events;cos(#theta);#events", 40, -1.0, 1.0);
  //histograms holding pull distributions
  //using the inverse Hessian matrix
  TH1D* hc0pull1 = new TH1D("hc0pull1", "Inverse weighted Hessian matrix [SumW2Error(false)];Pull (c_{0}^{fit}-c_{0}^{gen})/#sigma(c_{0});", 20, -5.0, 5.0);
  TH1D* hc1pull1 = new TH1D("hc1pull1", "Inverse weighted Hessian matrix [SumW2Error(false)];Pull (c_{1}^{fit}-c_{1}^{gen})/#sigma(c_{1});", 20, -5.0, 5.0);
  //using the correction with the Hessian matrix with squared weights
  TH1D* hc0pull2 = new TH1D("hc0pull2", "Hessian matrix with squared weights [SumW2Error(true)];Pull (c_{0}^{fit}-c_{0}^{gen})/#sigma(c_{0});", 20, -5.0, 5.0);
  TH1D* hc1pull2 = new TH1D("hc1pull2", "Hessian matrix with squared weights [SumW2Error(true)];Pull (c_{1}^{fit}-c_{1}^{gen})/#sigma(c_{1});", 20, -5.0, 5.0);
  //asymptotically correct approach
  TH1D* hc0pull3 = new TH1D("hc0pull3", "Asymptotically correct approach [Asymptotic(true)];Pull (c_{0}^{fit}-c_{0}^{gen})/#sigma(c_{0});", 20, -5.0, 5.0);
  TH1D* hc1pull3 = new TH1D("hc1pull3", "Asymptotically correct approach [Asymptotic(true)];Pull (c_{1}^{fit}-c_{1}^{gen})/#sigma(c_{1});", 20, -5.0, 5.0);
 
  //number of pseudoexperiments (toys) and number of events per pseudoexperiment
  constexpr unsigned int ntoys = 500;
  constexpr unsigned int nstats = 5000;
  //parameters used in the generation
  constexpr double c0gen = 0.0;
  constexpr double c1gen = 0.0;
 
  // Silence fitting and minimisation messages
  auto& msgSv = RooMsgService::instance();
  msgSv.getStream(1).removeTopic(RooFit::Minimization);
  msgSv.getStream(1).removeTopic(RooFit::Fitting);
 
  std::cout << "Running " << ntoys*3 << " toy fits ..." << std::endl;
 
  // M a i n   l o o p :   r u n   p s e u d o e x p e r i m e n t s
  //----------------------------------------------------------------
  for (unsigned int i=0; i<ntoys; i++) {
    //S e t u p   p a r a m e t e r s   a n d   P D F
    //-----------------------------------------------
    //angle theta and the weight to account for the acceptance effect
    RooRealVar costheta("costheta","costheta", -1.0, 1.0);
    RooRealVar weight("weight","weight", 0.0, 1000.0);
 
    //initialise parameters to fit
    RooRealVar c0("c0","0th-order coefficient", c0gen, -1.0, 1.0);
    RooRealVar c1("c1","1st-order coefficient", c1gen, -1.0, 1.0);
    c0.setError(0.01);
    c1.setError(0.01);
    //create simple second-order polynomial as probability density function
    RooPolynomial pol("pol", "pol", costheta, RooArgList(c0, c1), 1);
 
    //G e n e r a t e   d a t a   s e t   f o r   p s e u d o e x p e r i m e n t   i
    //-------------------------------------------------------------------------------
    RooDataSet data("data","data",RooArgSet(costheta, weight), WeightVar("weight"));
    //generate nstats events
    for (unsigned int j=0; j<nstats; j++) {
      bool finished = false;
      //use simple accept/reject for generation
      while (!finished) {
        costheta = 2.0*rnd->Rndm()-1.0;
        //efficiency for the specific value of cos(theta)
        double eff = 1.0;
        if (acceptancemodel == 1)
          eff = 1.0 - 0.7 * costheta.getVal()*costheta.getVal();
        else
          eff = 0.3 + 0.7 * costheta.getVal()*costheta.getVal();
        //use 1/eff as weight to account for acceptance
        weight = 1.0/eff;
        //accept/reject
        if (10.0*rnd->Rndm() < eff*pol.getVal())
          finished = true;
      }
      haccepted->Fill(costheta.getVal());
      hweighted->Fill(costheta.getVal(), weight.getVal());
      data.add(RooArgSet(costheta, weight), weight.getVal());
    }
 
    //F i t   t o y   u s i n g   t h e   t h r e e   d i f f e r e n t   a p p r o a c h e s   t o   u n c e r t a i n t y   d e t e r m i n a t i o n
    //-------------------------------------------------------------------------------------------------------------------------------------------------
    //this uses the inverse weighted Hessian matrix
    std::unique_ptr<RooFitResult> result{pol.fitTo(data, Save(true), SumW2Error(false), PrintLevel(-1), EvalBackend("cpu"))};
    hc0pull1->Fill((c0.getVal()-c0gen)/c0.getError());
    hc1pull1->Fill((c1.getVal()-c1gen)/c1.getError());
 
    //this uses the correction with the Hesse matrix with squared weights
    result = std::unique_ptr<RooFitResult>{pol.fitTo(data, Save(true), SumW2Error(true), PrintLevel(-1), EvalBackend("cpu"))};
    hc0pull2->Fill((c0.getVal()-c0gen)/c0.getError());
    hc1pull2->Fill((c1.getVal()-c1gen)/c1.getError());
 
    //this uses the asymptotically correct approach
    result = std::unique_ptr<RooFitResult>{pol.fitTo(data, Save(true), AsymptoticError(true), PrintLevel(-1), EvalBackend("cpu"))};
    hc0pull3->Fill((c0.getVal()-c0gen)/c0.getError());
    hc1pull3->Fill((c1.getVal()-c1gen)/c1.getError());
  }
 
  std::cout << "... done." << std::endl;
 
  // P l o t   o u t p u t   d i s t r i b u t i o n s
  //--------------------------------------------------
 
  //plot accepted (weighted) events
  gStyle->SetOptStat(0);
  gStyle->SetOptFit(0);
  TCanvas* cevents = new TCanvas("cevents", "cevents", 800, 600);
  cevents->cd(1);
  hweighted->SetMinimum(0.0);
  hweighted->SetLineColor(2);
  hweighted->Draw("hist");
  haccepted->Draw("same hist");
  TLegend* leg = new TLegend(0.6, 0.8, 0.9, 0.9);
  leg->AddEntry(haccepted, "Accepted");
  leg->AddEntry(hweighted, "Weighted");
  leg->Draw();
  cevents->Update();
 
  //plot pull distributions
  TCanvas* cpull = new TCanvas("cpull", "cpull", 1200, 800);
  cpull->Divide(3,2);
  cpull->cd(1);
  gStyle->SetOptStat(1100);
  gStyle->SetOptFit(11);
  hc0pull1->Fit("gaus");
  hc0pull1->Draw("ep");
  cpull->cd(2);
  hc0pull2->Fit("gaus");
  hc0pull2->Draw("ep");
  cpull->cd(3);
  hc0pull3->Fit("gaus");
  hc0pull3->Draw("ep");
  cpull->cd(4);
  hc1pull1->Fit("gaus");
  hc1pull1->Draw("ep");
  cpull->cd(5);
  hc1pull2->Fit("gaus");
  hc1pull2->Draw("ep");
  cpull->cd(6);
  hc1pull3->Fit("gaus");
  hc1pull3->Draw("ep");
  cpull->Update();
 
  return 0;
}