[ROOT] Meaning errors TGraph::Fit

From: Dario Motta (motta@mpi-hd.mpg.de)
Date: Fri Sep 19 2003 - 15:29:28 MEST


Dear rooters,
I have my data in form of a TGraph object and I fit them with a
user-defined function, indeed a simple double exponential function
defined as:
total = new TF1("total","expo(0)+expo(2)",5,100).
I don't have so far a good estimation of the data-points error, but I
surprisingly found that the errors on the fit parameters estimated by
MINUIT (called by TGraph::Fit) are perfectly "reasonable" and independent
on the normalization of the graphic (i.e. on any arbitrary weight factor
with which I multiply my y-values). In addition I find that the errors on
the parameters printed out by MINUIT on the screen (which indeed do depend
on normalization factors!) are different from those returned by
fift->GetParError(), that I use.
I conclude that some special algorithm might be called to estimate default
errors and to calculate parameters errors. Is my interpretation correct?
If so, can I have some more information on the algorithm implemented?

I use ROOT 3.04 on a linux PC.

Thanks,

Dario Motta



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