Hi,

I am using some Roofit to fit my data.

By default I was using an unbinned fit and everything seems ok except that a single fit takes about 5/6 minutes

to finish (and this for toys studies is not optimal for me…).

I tried with a binned fit. given a dataset

`RooDataSet data_b("data_b","data_b",RooArgSet(recoJetMass,weight),RooFit::WeightVar("weight"));`

I created a

`RooDataHist hist_b("hist_b","hist_b",RooArgList(recoJetMass),data_b);`

and fit with

`RooFitResult *fitresult=model.fitTo(hist_b,RooFit::Strategy(1),RooFit::Save(true),RooFit::Minos(false));`

I have 8 free parameters and the statistic is pretty large (45 bins. the less populated bin has ~2000 entries)

and so I expected to not see large difference between binned and unbinned fit result.

But I observe that the two fits converge on two different minima: the binned fit gets the “worst” minimum

( much larger chi2/ndf). If I set the initial parameters of the binned fit identical to the parameters obtained from unbinned fit it still moves into the other minimum. This is a bit puzzling.

is there something wrong in what I am doing?

cheers,

Francesco