//
pull( Int_t nit = 10000, Int_t nrnd = 100, Int_t nbins = 40 )
{

  Float_t sig, mean;


  // create histo
  TH1F * h4 = new TH1F("h4","Random Gauss",nbins,-4,4);
   h4->SetMarkerStyle(21);
   h4->SetMarkerSize(0.8);
   
  // hist for sigma and pull
  TH1F * sigma = new TH1F("sigma","sigma from gaus fit",50,0.5,1.5);
  TH1F * pull = new TH1F("pull","pull from gaus fit",50,-4.,4.);

  // make nice canvases
    TCanvas* c0 = new TCanvas("gauss","gauss",0,0,300,200);
    c0->SetGrid();
  // make nice canvases
    TCanvas* c1 = new TCanvas("Result","Sigma-Distribution",0,300,600,400);
    c0->cd();


  for ( i =0; i<nit; i++ ) {

    // reset histo contents
    h4->Reset();

    // fill histo
    for ( Int_t j = 0; j<nrnd; j++ ) {
      h4->Fill(gRandom->Gaus());   
    }

 // select one of the two lines below
    // h4->Fit("gaus","lq");    // likelihood fit
     h4->Fit("gaus","q");     // Chi2 fit

     if (!(i%100)) {
      h4->Draw("ep");
      c0->Modified();
      c0->Update();
    }

    // get sigma from fit
    TF1 *fit = h4->GetFunction("gaus");
    sig = fit->GetParameter(2);
    mean= fit->GetParameter(1);
    sigma->Fill(sig);
    pull->Fill(mean/sig * sqrt(nrnd));
  }
      c1->cd();
      pull->Draw();

}