Feature: resampling to uniform distribution

[bbullard]

There are two resampling methods already implemented (see documentation) that allow different populations of jets to be resampled to match a reference population distribution.

For the task of jet pt/mass regression, it is important to reweight the pt/mass distribution to be uniform. To implement this, a new target option named "uniform" can be supported that uses the same counter/pdf methods, but constructs a flat histogram on the fly based on the specified binning. Some additional protections should be imposed to prevent sampling in empty bins and limits on the number of resamples of a given jet.

[nikitapond]

I have a feeling that any uniform technique will have to be incredibly aggressive, either upsampling a huge amount or only using a tiny fraction of the available jets, unless you're starting with samples with relatively flat distributions, or only running UPP over a select region of phase space.
If this is still okay, then I think this is quite an easily implementation, I can try and get it done over the weekend.

[samvanstroud]

Is it really the case that the for target distributions need to be uniform for resgressions? Is this in the literature somewhere? I'd be interested to read about it more. I do agree with @nikitapond that there will for sure be a trade off here with stat loss

[bbullard]

Hi @nikitapond, thanks for volunteering. Functionality that allows the user to control how aggressively to resampling towards uniform would be the most complete. That being said, I realize now that if one wants to just have an increase in statistics in a certain region, this can already be done manually by defining separate regions for parts of a distribution that should be upsampled. This is probably enough to do the performance studies I have in mind, which could better motivate this feature. @samvanstroud I am not aware of anything yet in the literature about this, but naively the regression should perform worse in data that is underrepresented in the training set.

[nikitapond]

Okay, let me know if changing the binning doesn't work.

[samvanstroud]

It's an interesting question, I would have thought the performance was more dictated by the absolute statistics rather than relative. In other words, if you flattened a distribution with tails I would expect that you for sure would get worse in the area with reduced stat, and I'm also not 100% confident that you would improve much in the tails (where you have the same absolute number of training examples, but they now make up a bigger fraction of the overall training set).

Another suggestion would be to apply loss weights to effectively flatten the distribution without loosing and statistics. I recently added this to the classification task in salt, it would be pretty straightforward to add it also for the regression.