Hi all,
I never thought of background as "noise" - to me the noise should be
defined as the esd of the background points. You could get that by
averaging 10 or 20 background points and getting the standard deviation.
I'm told that for typical counting statistics, that should be close to
the square root of the background, so Mike's idea of using that square
root seems reasonable.
- Kurt
-----Original Message-----
From: Alan Hewat [mailto:[EMAIL PROTECTED]
Sent: Tuesday, February 19, 2008 3:10 AM
To: rietveld_l@ill.fr
Subject: RE: advice on new powder diffractometer
> So my question remains: what is the definition of signal to noise
ratio
> that is accepted for powder diffraction?
Why does it matter? A higher Bragg/Background ratio does not necessarily
mean better data if counting statistics are poor. Exaggerating slightly
:-) consider a single peak with a ratio of 100/1 compared to a peak with
a
ratio 10000/1000. The second measurement will give the lowest error, not
the first which has a much higher signal/noise. And you measure lots of
points on a slowly varying background, so you have a much better
estimate
of background than the normal error of a single point. Please don't
encourage people to simply maximise "signal/noise".
Similarly, low profile R-factor's can be obtained with low resolution
data
and high background. That does not mean that low resolution data
produces
smaller errors in structural parameters.
I worry about people treating measurement and refinement as black boxes
with simplified measures of quality such as R-factors, signal-to-noise
etc. You have to look at the physical reality of the model and the
estimated errors in its parameters, while not cheating by removing data
that doesn't fit for unknown reasons, adding too much "a priori"
information such as constraints, or throwing in extra garbage parameters
to improve the R-factors.
______________________________________________
Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE
<[EMAIL PROTECTED]> +33.476.98.41.68
http://www.NeutronOptics.com/
______________________________________________
