From: Ariel Goobar (ariel@physto.se)
Date: Tue Dec 09 2003 - 04:59:57 PST
Hi Chris et al,
Serena is working on finishing some important details of
I-band paper: redoing the LC fits with the Sofie J-band
K-corrections for the one point for 99Q (thanks Chris for the
transmission
curves!) and recomputing the
chisquare for the color-color diagrams in section 6 corresponding to Fig
17, where we concluded the numbers in the text are not really what we
want to use to test the IG dust hypothesis. In the mean time,
I thought I would comment on your comment below, with respect to the way
that the systematic uncertainty on trhe high-z LC fit is done, see
below.
>>
>> Moreover given the large difference in chisq between the best fit one
>> and
>> the next (at least for 2000fr and 1999ff), I do not think is really
>> fair
>> to average together the results up to chisq_min+3.
>> As you said, this will not change the results anyway, but I don't
>> think
>> averaging results from "worse" fits, would give a more robust
>> estimate of
>> the maximum.
>>
>
> I'm not sure and I'd like to explore this point further. Ask yourself
> the following question. Why would you use one sample to compute the
> first moment of a distribution and then a larger sample to compute a
> higher order moment. This is what you have done. You have used one
> point (the one with the lowest chi-sqaure) to compute the "average",
> but you use a larger sample (the ones with chisq_min+3) to compute the
> RMS. I'd argue that you should the same sample to do both. I.e
> compute the mean, median, standard deviation) from the same sample.
Let me give you another analogy, that I think is closer to what
Serena has done. Imagine that, instead of using the chi2
minimum to give our best fit omega-lambda cosmology, we would do
what you propose: compute the "mean" cosmology from all the
solutions that are within chi2_min +3. Wouldn't that be a very
odd procedure? The "orthodox" thing is to find your chi2 minimum
and establish your parameter uncertainty by looking at chi2_min +- 1,
for the 68% CL 1-dim case. This is exactly how Serena is trying
to assess the SYSTEMATIC uncertainty (note, not RMS/STATISTICAL
uncertainty) from her template "grid search" fit to the data.
Cheers,
Ariel
___________________________________________________________________
Ariel Goobar (www.physto.se/~ariel)
Department of Physics, Stockholm University
AlbaNova University Center, SE-106 91 Stockholm, SWEDEN
tel: +46 8 55378659 fax: +46 8 55378601
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