Re: [ROOT] chisquare

From: Eddy Offermann (eddy@rentec.com)
Date: Mon May 21 2001 - 19:30:21 MEST


Hi Mat,

With the L option you use the Poisson distribution to calculate the
likelihhod function, default is the Gaussian distribution which
results in the minimization of a chi-square.

The Poisson Likelihood function has not a problem with a bin
contents of zero.

see for instance "data analysis" by Siegmund Brandt , page 196, example 7.4

The comment "L" Use Loglikelihood method (default is chisquare method)
in the code is confussing because we always use a Loglikelihood method


> X-Authentication-Warning: oxpc01.fnal.gov: msmartin owned process doing -bs
> Date: Mon, 21 May 2001 11:25:06 -0500 (CDT)
> From: Matthew Martin <msmartin@fnal.gov>
> To: roottalk@pcroot.cern.ch
> cc: Todd Huffman <t.huffman1@physics.ox.ac.uk>
> Subject: [ROOT] chisquare
> MIME-Version: 1.0
> X-Filter-Version: 1.3 (wren)
> 
> Hi Rene, 
> 	When I do something like:
> 
> hist_bp_PtResidWeight->Fit("gaus","L");
> 
> how is the resulting chisquare defined? Ie how does it deal with the
> possibility of some bins with zero events in them?
> 
> cheers
> 
> Mat
> 

Eddy A.J.M. Offermann
Renaissance Technologies Corp.
Route 25A, East Setauket NY 11733
e-mail: eddy@rentec.com
http://www.rentec.com



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