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);
}
This archive was generated by hypermail 2b29 : Sat Jan 04 2003 - 23:51:11 MET