Re: Treatment of uncertainties from the TFractionFitter

Hello Ashish,

On 1 April 2010 11:12, Ashish Kumar <ashishk_at_fnal.gov> wrote:
> Hello,
>    I am using TFractionFitter to fit the data distribution with the
> templates from Monte Carlo simulation to extract their respective fractions.
>  The Fitter gives us the fractions with the errors. The problem is how to
> disentangle the  error into statistics and systematics parts. The error must
> have some part as statistical, but, clearly it has systematics too since it
> varies the shape of the templates while fitting. Can you please help me with
> this?

As I understand it TFractionFitter will change the central value of each bin if that change will lead to a better fit. By how much it moves it depends on the statistical uncertainty on that bin of the template. This way our lack of knowledge about the central value of a bin is taken into account. If all templates were made with infinite statistics then TFractionFitter would not move the central value of each bin, I believe.

IMHO the only "systematic" uncertainty which is then already part of the uncertainty on the fraction returned by TFractionFitter is the one often referred to as "MC statistics".

It should be easy to check this with a little toy MC. Generate two pairs of templates, once using moderate statistics and once with "huge" statistics. Then generate lots of pseudo datasets with a known fraction of template A and template B. Now if you fit your pseudo data with the pair of template with moderate statistics TFractionFitter will move around the central value of some bins. If you use the templates with "huge" statistics it should stop doing it(or at least at a much smaller rate).

Maybe an expert on this topic could comment?

Let me know what you find out,
Tim

ps. Take these ideas with a pinch of salt, I am not an expert!

-- 
http://tim.jottit.com/