Hello all,

A quick question regarding the background subtracted R values:

In the .LST file, GSAS gives two sets of values for wRp and Rp: "Fitted" and
"- Bknd." I understood the Fitted values to be calculated including the
background, and the -Bknd to be calculated with the background removed from
both the calculated and observed patterns. Is this correct?

I notice in some refinements I am doing that the - Bknd values are smaller
than the Fitted values which is not what I would have expected.

Thanks in advance,

Matt

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Matt Beekman
Department of Physics
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On Fri, Mar 6, 2009 at 2:55 PM, Brian H. Toby <brian.t...@anl.gov> wrote:

> I am not sure what all the fuss on background subtraction is all about, as
> long as the uncertainties in the data are unchanged. We have to model
> background in all powder methods and add it to the computed pattern. If you
> subtract something from the obs pattern before you fit, the only things that
> changes is the normalization term in the R factors. Since the purpose of the
> weighted R-factor is to measure the quality of different models to the same
> data, this really makes no difference. The unweighted r-factor is pretty
> much meaningless, so again who cares?
> I don't love the idea of modifying data, rather than applying a correction
> to a calculated value -- observations should be sacrosanct, IMHO, but having
> said that, more than once have I applied an absorption correction directly
> to measured intensities.
>
> The important point is that if you measure 100 counts at a point, the
> uncertainty in that is 10. If you decide that 95 of those counts are
> background, go ahead and subtract them if you must, but the uncertainty on
> the observation remains 10, and does not drop to sqrt(5) -- this means you
> must compute uncertainties before subtracting and use a Rietveld code that
> will accept "xye" input, as Pam pointed out.
>
> Brian
>
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