Question about error from TFractionFitter

I am playing with TFractionFitter and trying to understand the uncertainty in the fit parameter. I generated a “data” histogram with 3 Gaussian components, then fit this histogram to the three Gaussian. I then repeated the fit 10000 times and plot the difference between the input fraction and fit result normalized by the uncertainty.

What I found is that for the first two components, the normalized difference distribution can be described with a gaussian distribution with a sigma of 1.

But the third component has a sigma way less than 1.

Is there something I did wrong, or is TFractionFitter overestimating the error on the third component.
toyMC.c (3.3 KB)


This is a known problem of TFractionFitter, there is a note written some time ago, see Figure 1 in the note illustrates well the problem.

See as well the discussion in


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