fit with asymmetric errors data

From: Antonello Sbrizzi <asbrizzi_at_nikhef.nl>
Date: Mon, 11 Sep 2006 15:35:23 +0200 (MET DST)

Dear Rene' and Fons,

  I am a ROOT user and I recently experienced the following problem concerning the treatment of errors.

  I am fitting a set of data with asymmetric errors and retrieve the parameter errors calculated by MINOS.

  I would expect that MINOS is also able to return an asymmetric error on the fitting parametres, but I do not manage to do that. I wonder if Minuit is able to treat the input asymmetric error distribution.   

  Here is the simple example of code I am using:

{

gStyle->SetOptFit();

gROOT->Reset();

c1 = new TCanvas("c1","A Simple Graph with error bars", 400, 800); c1->Divide(1, 3);

c1->cd(1);

c1_1->SetFillColor(42);
c1_1->SetGrid();
c1_1->GetFrame()->SetFillColor(21);
c1_1->GetFrame()->SetBorderSize(12);

Int_t n1 = 2;

Double_t x1  [n1] = {1, 2};
Double_t exl1[n1] = {0, 0};
Double_t exh1[n1] = {0, 0};

Double_t y1  [n1] = {4.1, 3.3};

Double_t eyl1[n1] = {1.3, 1.6};
Double_t eyh1[n1] = {1.5, 1.9};

gr1 = new TGraphAsymmErrors(n1, x1, y1, exl1, exh1, eyl1, eyh1);

gr1->SetTitle(0);
gr1->SetMarkerColor(4);
gr1->SetMarkerStyle(21);
gr1->Draw("AP");

TF1 *f1 = new TF1("f1","[0]*exp(-[1]*x)", 1, 2);

f1->SetParameter(0, 1);
f1->SetParameter(1, 1);

//gr1->Fit(f1, "ve");
//gr1->Fit(f1, "vel");

gr1->Fit(f1, "vell");

//gr1->Fit("pol0", "ve");

}

Thanks,

Antonello Sbrizzi

-- 

  Antonio Sbrizzi

NIKHEF                            e-mail:    asbrizzi_at_nikhef.nl
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Received on Mon Sep 11 2006 - 15:35:32 MEST

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