Re: ISIS: loading data with proper statistical error

> If your concern in your last question is whether in rebinning, errors are propagated by 
> something like adding quadrature, don't worry, they're not (unless you write that
> yourself).

Yes that was my concern. Thanks a lot for your clarifications.

Nicolas

Dr. Nicolas Barrière
***********************************************
barriere_at_email.domain.hidden510 - 643 - 4747
Space Sciences Lab
7 Gauss way
Berkeley – CA 94720
USA
***********************************************

On Nov 30, 2012, at 2:23 PM, Michael Nowak wrote:

> 
> On Nov 30, 2012, at 12:49 PM, Nicolas Barrière wrote:
> 
>> Now, and it's a whole different discussion, how do you think I should calculate these errors?
>> A lot of bins contain less that 10 counts, so Gaussian statistics (sqrt(N)) would be totally underestimated. 
>> Poisson statistics would be more correct. Is there a build-in function to calculate the errors (something like what Gherels (1986) described)?
>> 
>> With Poisson errors in each bin, should I worry when I rebin the data that the errors will end up over-estimated?
> 
> If your concern in your last question is whether in rebinning, errors are propagated by 
> something like adding quadrature, don't worry, they're not (unless you write that
> yourself).
> 
> See the help file for "set_fit_statistic".  You get the usual choice of Chi^2 by data
> (i.e., Gaussian, the default), Chi^2 by model, Gehrels, Cash, Least Squares.  
> 
> In rebinning, ISIS calculates the new counts in the new bin, and then calculates the 
> new error from that. So, for example, in the Gehrels formula of:
> 
> 	1 + sqrt(Counts+0.75)
> 
> you only get that 1 & 0.75 once, not in quadrature from all the bins that made up the 
> new bin.  Counts is the rebinned counts.
> 
> I usually use straight sqrt(Counts), i.e., the default.  I'm less religious about trying to 
> never  bin data and trying to worry too much about going into the low counts regime.  
> But I  tend to look at bright things, and I tend to worry more about instrumental effects 
> (in  which case binning is good, under the hope that one averages out systematics to 
> some degree).
> 
> The few times I don't end up binning, I usually use the Cash statistic.
> 
> Cheers,
> 
> Mike
> 
> 
> 
> 

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