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Shape variations with HistFactory::HistoSys and normalization


Dear Rooters,

I am performing an analysis using the tool HistFactory in order to create the model to fit the data with.
In one of the samples we would like to insert a systematic variation to the histogram template. We would like this systematic to affect only the shape of the template and not its normalization.

At the moment we are implementing it with a HIstFactory::HistoSys object, providing the +/-1 sigma variation histograms to the method.

These variational histograms are created separately reweighting the MonteCarlo sample we are using to produce the template, but this method does not guarantee that the nominal histograms have the same normalization. Are the three histograms normalized inside the HistFactory classes before the morphing interpolation is performed, or should we make sure ourselves that their normalization is the same before passing them to the HistoSys object?

Thank you very much in advance for your help,



@Hageboeck, can you please help here?

Thank you so much,


Hello @simeloni,

No, they are not normalised. I would suggest to do something like this:

nominal = THxxxx;

In this way, you have a shape-only variation.


Thanks a lot @Hageboeck
I will implement this!



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