Fitting turn-on curve

ROOT
#1

Hi,

I’m trying to do a fit to a turn-on curve by using TMath:ERF. This works pretty well, however I have one turn-on curve where the fit is not working at all. This is what I get:

root [13] to_all->Fit(“fit”)
Warning in TH1F::Fit: Abnormal termination of minimization.
FCN=nan FROM MIGRAD STATUS=CALL LIMIT 5010 CALLS 5011 TOTAL
EDM=nan STRATEGY= 1 NO ERROR MATRIX
EXT PARAMETER CURRENT GUESS STEP FIRST
NO. NAME VALUE ERROR SIZE DERIVATIVE
1 p0 nan 4.80000e+00 nan nan
2 p1 nan 2.40000e-01 nan nan
3 p2 nan 2.10000e-01 nan nan
(Int_t)4
Warning in TCanvas::ResizePad: Inf/NaN propagated to the pad. Check drawn objects.
Warning in TCanvas::ResizePad: c1 height changed from 0 to 10

The data points look OK, but I have no idea how to proceed and find the error. I’m assuming that something in the histogram give Minuit a headache, but I don’t know what.

Any ideas?
Cheers,
Carsten

#2

Carsten,

Please send a short script reproducing the problem

Rene

#3

Hi Rene,

just do line-by-line the commands written in script.C. For some reason it doesn’t work as a macro…

Cheers,
Carsten
script.C (141 Bytes)
histo.root (5.89 KB)
TurnOnFit.C (2.05 KB)

#4

Carsten,

Could you send me a working macro? something like:

{
gROOT->ProcessLine(".L TurnOnFit.C");
TFile f = new TFile(“histo.root”);
TH1F to_all = (TH1F*)f->Get(“to_all”);
to_all->Draw();
TF1 *fit3 = new TF1(“fit3”,turnon_func,10.,18.,3);
fit3->SetParameters(13.0,1.0,0.75);
to_tr->Fit(“fit3”);
}

with the right call to TurnOnFit

Also, you may be interested by a new function in TGraphAsymErrors

root.cern.ch/root/htmldoc/TGraph … ayesDivide

Rene

#5

Hi Rene,

the macro which you’ve just posted works fine with me - but only if you comment out the last line. However the last line “to_all->Fit(“fit3”);” leads to this funny error message which I don’t understand. … Do you want anothe histogram file which looks the same and which works?

Cheers,
Carsten
macro.C (344 Bytes)

#6

Carsten,

If you want to restrict the fit to the range where the function is defined, you must specify the option “r”, eg
to_all->Fit(“fit3”,“r”);

Rene

#7

Hi Rene,

thanks for your help! That did it.

Cheers,
Carsten