Re: ISIS: loading data with proper statistical error

On Nov 29, 2012, at 6:11 PM, Nicolas Barrière wrote:

> I just started to use ISIS one week ago, and although I've been steadily becoming more comfortable with its use, I'm now stuck on a problem that should be very simple.
> 
> When I load a spectrum using 
> id = data_load ("filename.pha");
> 
> it seems that the statistical errors are not calculated.
> 
> I noticed that when I was trying to plot the data, but also when I do:
> 
> data = get_data_counts(id);
> print (data.err);
> 
> 
> Am I missing something?

If I'm remembering this all correctly, the default is for ISIS to read the error column in the data file and use that.  So perhaps the data is not properly stored in the column.  

There are a few caveats, however:

1) There is a minimum statistical error controlled by a global variable, Minimum_Stat_Err, which is normally set to 1 (so bins with 0 counts don't
get set to 0 error).  I normally reset that to 1e-30 for various reasons.  That
could change things.

2)  ISIS does not keep the grouping of the data set - after reading it in, 
ISIS goes back to unbinned data.

    group(id;min_sn=4.5,bounds=0.5,unit="kev");
    notice_values(id,0.5,8;unit="kev");

will get you something like the usual minimum 20 counts/bin (assuming no
background), and then notice the 0.5-8 keV range.  Once you rebin, ISIS
applies the "standard" rules for the statistical error (usually square root of
counts, but you can override this behavior, and I do, for example, when using
a separate spectrum as a "correction file" to the background).

So, if the error bars in the pha file aren't what you expect, and you haven't
grouped the data after reading it in, it's possible to have the error bars be
not what you expected.  Regrouping the data or changing the pha file error
column should change it.

You can do get_data_counts/put_data_counts combinations to override things,
but you probably want to avoid that since that itself will be overridden if you do
any rebinning of the data during analysis.  (And that's one of the main features 
we use of ISIS - the ability to rebin the data during analysis.)

If that doesn't do it for you, send me a copy of the data with a little bit more detail
about how it's deviating from expectations, and I'll take a closer look.

Cheers,

Mike Nowak




----
You received this message because you are
subscribed to the isis-users list.
To unsubscribe, send a message to
isis-users-request_at_email.domain.hiddenwith the first line of the message as:
unsubscribe