Hi Rene,
Here is the code:
test()
{
h=new TH1F("h","",100,0,1);
for(int i=0;i<10000;i++){
Double_t x1=gRandom->Gaus(0.3,0.05);
h->Fill(x1);
}
h->Draw();
TF1 *f=new TF1("f","gaus",0,1);
//f->FixParameter(1,0.3);
f->SetParLimits(1,0.3,0.3);
h->Fit(f);
}
When FixParameter is used, the result is: FCN=32.5374 FROM MIGRAD STATUS=CONVERGED 37 CALLS 38 TOTAL
EDM=1.80376e-09 STRATEGY= 1 ERROR MATRIX ACCURATE EXT PARAMETER STEP FIRST NO. NAME VALUE ERROR SIZE DERIVATIVE 1 Constant 8.10627e+02 1.00062e+01 2.29596e-02 -6.79730e-06 2 Mean 3.00915e-01 fixed 3 Sigma 4.90648e-02 3.54994e-04 5.52081e-06 -2.63700e-02 And about the SetParLimits: FCN=38.4282 FROM MIGRAD STATUS=CONVERGED 37 CALLS 38 TOTAL EDM=2.83056e-07 STRATEGY= 1 ERROR MATRIX ACCURATE EXT PARAMETER STEP FIRST NO. NAME VALUE ERROR SIZE DERIVATIVE 1 Constant 8.00170e+02 9.92677e+00 2.45814e-02 -8.89375e-05 2 Mean 2.99435e-01 fixed 3 Sigma 4.96739e-02 3.64274e-04 6.04274e-06 -3.11864e-01
In both cases, you can see the value of Mean is changed.
How can I really fix it?
I am using 5.10/00.
Thanks!
Cheers!
Xianguo
On 6/12/06, Rene Brun <Rene.Brun_at_cern.ch> wrote:
>
> Could yous end the SHORTEST POSSIBLE RUNNING script reproducing the
> problem?
>
> Rene Brun
>
> Xianguo (Hsien-Kuo) LU wrote:
> > Hi, rooters!
> >
> > I want to fit a histogram with fixed parameters.
> > I did the following:
> >
> > f=new TF1("f","gaus",0,1);
> >
> > f->SetParLimits(1,1.1156,1.1156)
> > or
> > f->FixParameter(1,1.1156)
> >
> > but after fitting, the parameter changed.
> > Can somebody tell me why and how to solve it?
> >
> > Thanks in advance!
> >
> > Cheers!
> > Xianguo LU
> >
> >
> >
> > --
> > Take flight into the sky, beyond the moon, beyond my mind.
> >
> > Xianguo LU (Hsien-Kuo LU)
> > Physics Department, Peking University
> > Beijing, China
> > Tel: 0086-10-62753888(o)
>
>
-- Take flight into the sky, beyond the moon, beyond my mind. Xianguo LU Physics Department, Peking University Beijing, China Tel: 0086-10-62753888(o)Received on Tue Jun 20 2006 - 12:07:40 MEST
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