Re: fitting different subranges with different functions

Your TGraphErrors has several points with a null error in y. This gives an infinite weight to these points and the fit does not converge when fitting with a poln that uses by default the linear fitter. You can force a fit with equal weights for all points by doing

   graph.Fit("pol7","w");
One can force a fit with Minuit (instead of the LinearFitter) by doing   graph.Fit("pol7","l"); or graph.fit("pol7","lw");

Rene Brun

Nevzat Guler wrote:
> Hello,
> I am having a difficulty in finding a nice fit to the data I
> am sending in attachment. I am mostly using pol7 for similar data in
> different but this one fails. (By the way fitting converges if I
> don't include errors to the graph (TGraph), but when I fit TGraphErrors it
> fails..)
> I have two questions:
> 1. Is there a flag that I can look when fit fails, so that I can try
> another function immediaetly in the program..
>
> 2.
> I am trying to find a way to fit different subranges with
> different functions but the demo multifit.C does not work for me. One of
> the things I tried by using this example is printed at the end of this
> email. The multifit.C example only uses parameters of fits from different
> subranges and using them as initial parameters it manages to find
> resulting combined fit. But what I need, I think, to fit different
> subranges and then combine resulting fits, merge them. One way I can think
> of to do this I can evaluate values of different fits and I can take their
> average in overlapping regions but it is not a good way to do this, I am
> wondering if there is a better and more reliable method..
> I will appreciate any suggestion.
>
> Thanks,
>
> Double_t el_par[19];
> TF1 *g1 = new TF1("g1","pol2",1,7);
> TF1 *g2 = new TF1("g2","pol7",4,20);
> TF1 *g3 = new TF1("g3","pol7",10,25);
> TF1 *total = new TF1("total","pol2(0)+pol7(3)+pol7(11)",1.1,29);
> total->SetLineColor(2);
> EL_scl2EL->Fit("g1","R");
> EL_scl2EL->Fit("g2","R+");
> EL_scl2EL->Fit("g3","R+");
> g1->GetParameters(&el_par[0]);
> g2->GetParameters(&el_par[3]);
> g3->GetParameters(&el_par[11]);
> total->SetParameters(el_par);
> total->SetParameters(el_par);
> total->FixParameter(0,0.0);
> EL_scl2EL->Fit(total,"R+");
>
>
>
> Nevzat Guler
Received on Fri Jul 13 2007 - 21:24:31 CEST