Hi Stephanie,
Note that the option "B" does (did) not exist in TGraph::Fit.
When fitting with predefined functions like poln,expo,gaus,
the inititial values of the parameters are automatically computed.
However, some time it may be useful to switch off this automatic
computation. I have added an option "B" in TH1::Fit and
TGraph::Fit to disable this automatic initial values.
The new option is now in CVS.
Rene Brun
On Thu, 15 Feb 2001, S. Escoffier wrote:
> Dear Rooters,
>
> I would like to report a bug in fitting a TGraphError:
> If I fit a TGraphError with a polynomial function, with the parameter 0 fixed
> to 0. ,
> then results are not in agreement with initials values. I mean, p0 is well
> fixed, but not equal to 0.
> If function f1 is described by
> TF1 *f1 = new TF1("f1","[0]+[1]*x",0.,5.);
> then it works.
>
> I'm working with 2.23.12 version on UNIX system.
> Sorry if this problem has been already reported.
>
> Stephanie.
>
>
> void test()
> {
> Float_t x[4]={1.,2.5,3.5,4.75};
> Float_t ex[4]={0.01,0.01,0.01,0.01};
> Float_t y[4]={0.,1.,2.,3.};
> Float_t ey[4]={0.01,0.01,0.01,0.01};
>
> TGraphErrors* g1 =new TGraphErrors(4,x,y,ex,ey);
>
> TF1 *f1 = new TF1("f1","pol1",0.,5.);
> f1->SetParameter(0,0.);
> f1->SetParLimits(0,1,1);
>
> TCanvas* c1;
> c1 = new TCanvas("c1","c1",100,10,800,500);
> c1->cd();gStyle->SetOptFit(111);
> g1->Draw("AP");
> g1->Fit("f1","B");
>
> }
>
> Print on screen:
>
> FCN=2442.08 FROM MIGRAD STATUS=CONVERGED 18 CALLS 19 TOTAL
> EDM=4.66971e-08 STRATEGY= 1 ERROR MATRIX ACCURATE
>
> EXT PARAMETER STEP FIRST
> NO. NAME VALUE ERROR SIZE DERIVATIVE
> 1 p0 -2.75862e-01 fixed
> 2 p1 6.47669e-01 1.72543e-03 4.16384e-05 -1.77117e-01
>
>
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