Fit bias in HistFactory with MINOS error

[june0812]

Dear experts,

Maybe it is related with Fit bias in HistFactory.

I found that pull distribution with MINOS error produce a fitter bias.

The pull distribution shows a discontinuous shape at 0. Here is the code to reproduce the problem:
Histfactory_bias.cc (10.3 KB)

This code

1. make histogram templates and workspace
2. fluctuate the nuisance parameter by Normal distribution. Generate toy MC and fit
3. iterate (2) 100000 times and make pull distribution

The pull is calculated by the equation (27) in https://lucdemortier.github.io/assets/papers/cdf5776_pulls.pdf. The pull distribution becomes

You can see some bias near 0.

So, is it expected in Histfactory? Or do I make some mistakes?

Thank you

[mczurylo]

Hello @june0812,

I believe @jonas will be answer your question.

Cheers,
Marta