Possible to convert a RooFitResult to a TFitResult?

Roofit and RooStats
1

I would like to be able to make a ratio plot for the data and a fit in RooFit. It would of course be convenient to use TRatioPlot to do so, however the constructor for this

TRatioPlot::TRatioPlot(TH1* h1, Option_t* option = "", TFitResult* fitres = 0)

requires a TFitResult and RooFit’s RooAbsPdf::fitTo() returns a pointer to RooFitResult.

Is there any clean way to go from a RooFitResult object to a TFitResult object?

Additionally, if there is some very nice way to get a ratio plot in RooFit that I have missed that would be even better.

2

I think @moneta will know the answer.

3

Hi,

In principle this should be possible, almost all the information needed for TFitResult is in the RooFitResult, but
it might be not trivial.
Since you are doing a TRatioPlot for comparing an histogram with a function, this is a rather simple fit. The easiest solution is to convert your RooFit pdf to a TF1 using the asTF function and do a histogram fit to obtain a TFitResult

Lorenzo

4

@couet @moneta Thanks for the reply. I would like to try to follow up on this as it seems like this would also fix @poniatowska’s problem as RooAbsReal::asTF() returns a pdf, not the fit you want.

In the more general case where one has a RooFitResult then (in PyROOT) you’re suggesting to do this?

# model is our RooAbsPdf (let's say a RooGaussian)
# datahist is the RooDataHist that we fit model to
# x is our observable RooRealVar
# x_range is the fit range we care about

# roo_fit_result = model.fitTo(datahist, ...) # Assume something like this has been done
fit_result_func = model.asTF(ROOT.RooArgList(x))
hist = datahist.createHistogram('hist name', x)
# This gets us the TFitResult
fit_result = hist.Fit(fit_result_func.GetName(), 'ILS', '', x_range[0], x_range[1])

# Now fit_result_func should have the parameters of the fit
integral = fit_result_func.Integral(x_range[0], x_range[1])

If this is what you are suggesting, then I think this seems to work and can easily have a user wrapper function that handles all of this (however a quick test results in an error with illegal number of parameters from hist.Fit()). However, I am a bit concerned about the fact that RooFit and ROOT’s fits do NOT seem to give consistent results (c.f. my chi-square question). Do you have advice here?

5

See my answer in the other post.
Lorenzo

6

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