Parameters in line3Dfit.C

Roofit and RooStats
1

Hi,

I am having a look at the tutorials to try to understand how to use TVirtualFitter.
In the tutorials, the macro line3Dfit.C has the following lines:

min->SetParameter(0,“x0”,pStart[0],0.01,0,0);
min->SetParameter(1,“Ax”,pStart[1],0.01,0,0);
min->SetParameter(2,“y0”,pStart[2],0.01,0,0);
min->SetParameter(3,“Ay”,pStart[3],0.01,0,0);

And having a look at the class TVirtualFitter, the last two parameter are named vlow and vhigh.
Then I assume that that would mean that these are the limits that the parameter could take, if that is
correct, then why set both limits to 0? Also, is there any rule of thumb to decide which value use for the
third parameter value?

Thanks a lot in advance for the help,
Hermes

2

Hi,

If vlow and vhigh are equal to zero the parameters are considered unlimited.

You need also to specify the initial parameter value which must be your best guess as close as possible to the parameter value at the function minimum.
The other parameter is the starting step size or an approximate parameter error. If the error is not known, normally something of the order of 10% of the value or of the allowed range is fine. It is not a critical value, the minimizer normally finds the optimal value after the first iteration.

Best Regards

Lorenzo