Hi,
Some observations:
1) when specifying errors in x, your fit becomes non-linear in the parameters !!
Specifying 'W' option makes it a linear fit
2) Your data weights are very "challenging: do you really think
specifying y[6] is important when neighboring data points
y[4] and y[5] have each
a weight in the fit that is 2500 times larger ?
3) Your fit returns with the message that the matrix was not
positive-definite (probably related to 2) ), so results can
not be trusted.
Rene, I tried to play around with the scripts by fixing some parameters:
TF1 f1("f1","pol2");
f1.FixParameter(0,-3.22);
f1.FixParameter(1,0.349);
f1.SetParameter(2,-6.93469e-08);
graph->Fit("f1","LEM");
The parameters are indeed fixed but not at the expected values ??
Eddy
> X-Authentication-Warning: pcroot.cern.ch: majordomo set sender to
owner-roottalk@root.cern.ch using -f
> Date: Mon, 30 Sep 2002 12:06:03 +0200 (MET DST)
> From: Alexander Dietz <adietz@mickey.mpi-hd.mpg.de>
> To: root list <roottalk@pcroot.cern.ch>
> Subject: [ROOT] Problems with fiting
> MIME-Version: 1.0
>
> Hello Rooters,
>
> I have a problem with doing a fit with a TGraphErrors. In the program
> source (at the end of this mail) I want to perform a quadratic fit with
> 'TGraphErrors", using the errors as specified.
> When I use the fitting option 'W' (all errors set to 1) then the fit works
> very good, giving a sensefull result.
> But without the option 'W' the result of the fit is very, very bad and
> wrong.
> The correct result is, that parameter 1 is about 0.35.
> But with the option 'W' this parameter is around 10^7 !!!
>
> So what can I do to get a correct fit which is using the errors I have
> specified?
>
>
> Regards
>
> Alexander Dietz
>
>
> void prog(void)
> {
> float x[10];
> float y[10];
> float ex[10];
> float ey[10];
>
> x[0]= 7508.84 ;ex[0]=0.037;
> x[1]= 1680.41 ;ex[1]=0.022 ;
> x[2]= 2093.11 ;ex[2]=0.05 ;
> x[3]= 2259.89 ;ex[3]=0.157;
> x[4]= 2474.87 ;ex[4]=0.065;
> x[5]= 2569.45 ;ex[5]=0.473;
> x[6]= 3100.43 ;ex[6]=0.272;
> x[7]= 4574.82 ;ex[7]=0.147;
> x[8]= 6041.46 ;ex[8]=0.15 ;
>
> y[0]=2614.53 ;ey[0]=0.013;
> y[1]=583.191 ;ey[1]=0.002;
> y[2]=727.33 ;ey[2]=0.009;
> y[3]=785.37 ;ey[3]=0.08;
> y[4]=860.564 ;ey[4]=0.005;
> y[5]=893.40 ;ey[5]=0.005;
> y[6]=1078.62 ;ey[6]=0.1;
> y[7]=1592.54 ;ey[7]=0.013;
> y[8]=2103.53 ;ey[8]=0.013;
>
>
> TCanvas* c1=new TCanvas("c1","c1",10,10, 400,300);
> TGraphErrors* graph=new TGraphErrors(9,x,y,ex,ey);
>
> graph->Draw("A*");
> graph->Fit("pol2","LEM");
> graph->Draw("");
>
> double p0=graph->GetFunction("pol2")->GetParameter(0);
> double p1=graph->GetFunction("pol2")->GetParameter(1);
> double p2=graph->GetFunction("pol2")->GetParameter(2);
> }
>
>
Eddy A.J.M. Offermann
Renaissance Technologies Corp.
Route 25A, East Setauket NY 11733
e-mail: eddy@rentec.com
http://www.rentec.com
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