Re: [sdpd] Synchrotron powder diffraction

[Jonathan Wright <wright...@esrf.fr>]

On Sun, 9 Sep 2001 pstephens...@notes.cc.sunysb.edu wrote:

> I think that most people active in this field understand that chi squared
> is a measure of the statistical quality of a fit to data, and as such, it
> cannot be much less than unity.  If a computer code gives you a value of
> chi squared of 0.518  for a profile refinement, it must mean that there is
> something wrong with the way that the program was used.  And I would not
> think that any experienced user of powder diffraction software would accept
> chi squared of 0.518 at face value.

Using w_i=sqrt(y_i) is sometimes the root of all evil with respect to
"anomalous" chi^2 values, particularly when a lot of points have less
than 10 (real) counts. With a synchrotron the instrumental background
can be just too darned low :)

Regards, Jon


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