# Retrieve parameters errors from ROOT::Math::Minimizer

Dear Root experts!

I have two questions concerning ROOT::Math::Minimizer class:

1. how to retrieve the fitted parameters errors from ROOT::Math::Minimizer after minimization being complied?

2. min->SetLimitedVariable (0,“x”,0.5, 0.001, lower , upper).
Is that correct when lower==upper then parameter “x” is free (unfixed)?

Regards,
defo900

I’m sorry, forgot to add to question 2) : lower == upper == 0

Hi,

1. The fitted parameters can be retrieved from Minimiser::X()

2. For unlimited parameter you should use Minimiser::SetVariable and not SetLimitedVariable.
It is not a good idea to set the bounds to be equal. I think in this case the variable might be considered constant

Best Regards

Lorenzo

Dear Moneta

Thank you for answers.

1. The fitted parameters can be retrieved from Minimiser::X()
I know about this: there is your appropriate example here “https://root.cern.ch/doc/master/NumericalMinimization_8C.html”.
However my question was about parameters errors. I suspect it is the method Errors().
But looking at corresponding web page “https://root.cern.ch/doc/v606/classROOT_1_1Math_1_1Minimizer.html” I find very scant description:
"errors at the minimum"
and I’m not sure if this method returns exactly parameters errors.
1. I think in this case the variable might be considered constant.
In case of usual TMinuit one can set unfixed parameter using both lower and upper limits equal to zero:
gMinuit->DefineParameter(0,“x”,start,step,0.,0.);
So is the same true for the TMinuit2 method SetLimitedVariable()?

Regards
defo900

Hi,

• Yes Error() is the function to retrive the symmetric parameter uncertainties

• In the Minimiser interface you use SetVariables for the unbounded ones or SetLimitedVariables for the bounded ones. You should not set min=0 and max=0 in SetLimitedVariables

Best Regards

Lorenzo

Dear Moneta

Thanks a lot!

All the best!