I’m trying to perform an extended simultaneous fit of a pdf (gaussian signal + polynomial bkg) splitted in categories (using a RooSimultaneous).
According to the manual (v2.91, p17) I expect to obtain numbers of signal and bkg such that the sum, in each category, is equal to the number of generated events in each category.
This is exactly what I obtained if I fit the values of Nsig & Nbg (defined as RooRealVar) in each category.
Now, I would like to fit directly Nsig as a function of some parameters f interest. Nsig is now defined as a RooFormulaVar.
But when trying to perform such a fit, the result of the fit do not converge on values such that Nsig = Ngenerated - Nbg in any category. The difference is of the order of 2/1000.
Should I worry about it ? Is it an expected feature of the fit ? Why is the behaviour different between the two cases ?
Many thanks for the help.