# Likelihood fit in root and roofit

Hi,

Just curious, why the binned-likelihood fit gives gives different results using ROOT and ROOFIT

For ROOT i just fit the histogram as
h->Fit(“pdf”,“L”); // likelihood fit to 1D histogram"h" (1)

With roofit i use:
RooFitResult* fitr=pdf.fitTo(mc); //MC is roodatahist (2)

(1) and (2) should be exactly same, isn’t this true ?

Best,
Sanjay

Hi,

There are some small differences between the binned likelihood fit in ROOT and RooFit, in particular for what concerns the normalisation of the function and its evaluation in the bin.
Both methods are approximated because the compute the function at the center of the bin, however the RooFIt approximation is less precise and in some case can produce a bias larger than the ROOT one.
This is discussed also here

https://sft.its.cern.ch/jira/browse/ROOT-3874

Lorenzo

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