Re: different fit behaviour in ROOT and ROOFIT

Dear Lorenzo,

thanks a lot for having looked to my problem and for spotting the "guilty" fitter :)
Unfortunately, the discrepancy between Root and Roofit is something which I always observe, while performing fits to the invariant mass spectra, i.e. it is not something related to the specific histogram I had attached to the mail...
It's really a pity to give up the use of Roofit, since (before discovering the discrepancy with ROOT) I had the impression that the fit stability and the speed where really very good!

...in my opinion, it would be important to understand why roofit seems not to behave properly, since it is now widely used in the LHC experiments! Please, let me know if there are any news!

  Ciao and thanks again!

     Roberta

On Thu, 1 Sep 2011, Lorenzo Moneta wrote:

> Hi Roberta,
>
> Sorry for having looking late into this problem. I have investigated more, it required me some time and I have concluded that the RooFIt result is probably not right, while the ROOT is. The reason for the difference is still to be found.
> I have been using the same pdf definition of RooFit, by transforming the pdf in a TF1 and then fitted using ROOT and I get consistent results
> with your TestROOT macro. In particular if I use as input the RooFit parameters , I get clearly a smaller value of the likelihood function
> (meaning that is not optimal).
> Furthermore, when using a chi2 fit method (not maximum likelihood) which should work perfectly fine and give the same results in your case since the histogram bin errors are gaussian I get again the same result. ( NJPSI ~ 2200)
>
> So, something is probably wrong in the RooFIt fitting, it could be also a numerical problem not dealt correctly in the likelihood calculation in RooFit.
>
> I attach my macro, which uses the RooFit to build the model but ROOT for fitting
>
> Cheers,
>
> Lorenzo
>
>
Received on Fri Sep 02 2011 - 10:19:25 CEST