[RF] Fitting RooSimultaneous with Range option not accounting for range on the indexCat

[will-cern]

The expected behaviour here would be if I define a range on the indexCat, the fit should only involve the pdfs of the categories in the range if I specify Range("rangeName") as an option to the fitTo method.

Attached is a demo workspace containing a two-channel model with a floating norm factor on each channel. Ranges "r1" and "r2" have been defined on the category var which respectively only include one of the channels. However, running:

w->pdf("simPdf")->fitTo(*w->data("obsData"),RooFit::Range("r1"))

on the workspace shows that the floating parameter for both channels gets fit to.

simDemo.zip

[guitargeek]

Hi, thanks for opening this issue. Sorry that nobody was looking at this yet, I see that you already opened a Jira issue about this many years ago (I closed the Jira issue now as duplicate). Is this problem still relevant? I would start working on this then.

[guitargeek]

Just for the paper trail, here is Will's original text from the 2016 Jira issue 8304:

Hello,

I am trying to run a fit in a restricted range where one of the observables is a RooCategory. I have defined a range that contains the states I want to consider in the fit. I am trying to do the fit with pdf->fitTo(data,Range("myrange")). I have noticed that in RooAbsOptTestStatistic that the reduce method is called on the input data, which will correctly restrict the fit to just the data in my desired fit range, including the RooCategory observable. But there is no restriction placed on the discrete observables, whereas there is one on the continuous ones: https://root.cern.ch/gitweb/?p=root.git;a=blob;f=roofit/roofitcore/src/RooAbsOptTestStatistic.cxx;hb=01602ce4a5f22fde41f2543cfaa066e0c9007714#l301

Could the code at this location be extended to support restricting the range of discrete observables in the fit?

Thanks
Will

[guitargeek]

I'll also change the tag from "bug" to "feature request". It's not obvious that a discrete category can have a "range", because a range is over something that is ordered. A RooCategory represents discrete, unordered labels, so I'd say the "range" doesn't apply to them. What is the usecase for this range support? Is it for fitting only some channels in a RooSimultaneous? Maybe we should find another way to do this easily, some way that also makes conceptually more sense than exploiting the range mechanism.

What do you think?

[will-cern]

So I think it's well defined what we mean by the range of a discrete variable - each value is declared as either being in the range or not in the range. RooFit already supports this for RooCategory, so that's all well and good. I do think it makes sense then to think about the behaviour of fits when such a range is declared. It's actually possible my original jira issue might have been referring to a case where I had a PDF that wasn't a RooSimultaneous but was dependent on a RooCategory and it wasn't handled correctly, I'd have to look back at it and remind myself. But for sure in the case of the RooCategory being the index category of a RooSimultaneous I think it makes total sense for the RooSimultaneous to be 'reduced' to just the channels that are in the range.