How to draw Minuit contour?

Roofit and RooStats
1

Hello,
I have a fit function with two free parameters. For the fit I’m using user defined FCN which is given to Minuit for the minimization. At the end I’m using gMinuit->Contour(80,0,1); to draw 1 sigma confidence contour for parameter 1 vs parameter 2.

I would like to compare my contour to a contour in a article. My question is how to draw a confidence contour for two parameters, if I know values and errors of both parameters, and correlation coefficient between the parameters?
I was looking how to give those external parameters to Minuit and then call “Contour” for calculation points of ellipse.

Please help.

Thanks in advance,
David

2

Hi,

Minuit, to get the correct contour, uses the full likelihood (or chi2) function.
In your case, if you have just errors and correlation coefficient, you could assume that the log-likelihood (or chi2) is parabolic and derive the contour from that parabola.
For a given confidence level, the contour is obtained from the values for which the log-likelihood (or chi2) has a certain value.
See for example seal.web.cern.ch/seal/documents/ … nerror.pdf
or chapter 2 of this book, Data Analysis in High Energy Physics: A Practical Guide to Statistical Methods.

Best Regards

Lorenzo