Fit return warning message: large likelihood value


I’m using the PLR procedure with RooStat and RooFit and I have a very concerning issue.

I’m defining a likelihood with external constrains on some nuisance parameter and during the first fit of the PLR, I receive this message:

[#0] WARNING:Eval -- RooAbsPdf::getLogVal(Constraint_Neutrino) WARNING: large likelihood value: 2.5168e+07

I’m defining the external constrain in this way:

RooRealVar N(Form("N_%s", contrib_name), Form("Number of %s bkg events", contrib_name), Ncount, 0, Ncount*10);  

RooRealVar N0(Form("N0_%s", contrib_name),Form("Estimated number of %s bkg events from aux measurement", contrib_name), Ncount);         
RooRealVar sigmaN0(Form("sigmaN0_%s", contrib_name), Form("Absolute error on expected %s BG", contrib_name), activitySigma * Ncount);    

fw->factory(Form("Gaussian::Constraint_%s(N_%s, N0_%s, sigmaN0_%s)", contrib_name, contrib_name, contrib_name, contrib_name)); 

The value of Ncount is ~10-7 and sigmaN0 = 0.2* Ncount

Any idea where this could come from ?


Probably you are evaluating the constraints many sigma away and it is normal that the resulting log-likelihood is very large since the pdf (the gaussian constraint) is almost zero.
The message is not normally a problem if it is coming during the fit, since this can be normal that during the minimisation procedure such points are tested


The warning message is not giving enough information about the value that is tested. Is there a way to check that ?

You can try RooFit::Verbose(true) as an argument to fitTo(). If I remember correctly, it will print the current function value and the parameters that have been updated in each minimisation step.

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