[Ifeffit] What confidence limit is implied in the error bars for least-squares fitting in Athena

Bruce Ravel bravel at bnl.gov
Thu Oct 2 17:23:04 CDT 2008


On Thursday 02 October 2008 18:06:19 Matt Newville wrote:
> Does it?  I thought it used data_uncertainty=1 unless the uncertainty
> was explicitly specified.  Does Athena use a value determined from the
> sigma when merging data, or is something?   Should I read the Users
> Guide more closely?

Hmmmm... perhaps I am confused.

Athena uses Ifeffit's minimize function.  As you say, the data
uncertainty is set to one, so the diagonals of the covarience matrix
will be orders of magnitude too small.  My understanding is that
Ifeffit rescales the error bars on the variable parameters in the same
manner as the feffit function does -- that is, by the square root of
reduced chi-square.

In that case, the error bars are reported are 1-sigma error bars given
the assumption that the fit is, in fact, a good fit.

Or do I not understand what minimize is doing?  Or are there too many
possible meanings of the word "sigma" in this context such that we are
talking about different things?

B



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