Get value of PLL from ProfileLikelihoodCalculator

Roofit and RooStats
1

Hi!

Is there any way to ask the ProfileLikelihoodCalculator for the value of (the logarithm of ) the profile likelihood for a given value of the parameter of interest ( or -2log(lambda(mu)) in the usual terminology) ? It’s very easy to plot that for various mu by using the LikelihoodIntervalPlot but funnily enough I could not find any method to get the actual value for one specific mu ( as the name ‘ProfileLikelihoodCalculator’ is suggesting).

Thanks in advance,
Bjorn

2

Hi,

You can get the profile likelihood ratio function using LikelihoodInterval::GetLikelihoodRatio.
Below is an example on how to get the value of the profile likelihood ratio function for a given value of the parameter of interest (e.g. poi = 1)

  // create calculator from data set and ModelConfig
  ProfileLikelihoodCalculator pl(*data,*mc);
  pl.SetConfidenceLevel(0.683); // 68% interval
  LikelihoodInterval* interval = pl.GetInterval();

   // find the interval on the first Parameter of Interest
  RooRealVar* firstPOI = (RooRealVar*) mc->GetParametersOfInterest()->first();

  // get value of PLR for POI=1
  RooAbsReal * plr = interval->GetLikelihoodRatio(); 
  firstPOI->setVal(1);
  double val = plr->getVal(); 
  cout << "PLR  value at " << firstPOI->GetName() << " = 1  : " << val << endl;

Best Regards
Lorenzo

3

Thanks a lot! This is what I was looking for.

Bjorn