Re: Misleading chi-square

On Mon, Nov 17, 2003 at 15:01 -0500, John E. Davis wrote:
> Hi John,
> 
>    I noticed that when I tried to simultaneously fit many data sets
> extracted on a chip-by-chip basis, my chi-square dropped dramatically.
> The problem turned out to be due to large wavelength regions where
> there were no counts, and where the arf was identically zero (the
> wavelengths fell off the chip).  I can always ignore such ranges but
> that will become a logistical nightmare when juggling 15 or so data
> sets.  I though about using get/put_data_counts and get/put_arf to
> truncate the data and arf. However, the docs for put_data_counts seems
> to preclude this.  Any other suggestions? Perhaps a mechanism can be
> added to isis to handle this situation.  I suppose that I can use
> combine_data to work around it.
> 
> Thanks,
> --John


I'd suggest using ignore_list() to do something like this:

foreach (all_data)
{
   id = ();
   info = get_data_info (id);
   arf = get_arf (info.arfs[0]);
   ignore_list (id, where (arf.value <= 0));
}

where, for simplicity, I've assumed that the ARF grid matches
the data grid.

Thanks,
-John
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