Re: [sdpd] error measurements - optimal weighting schemes?

[Lachlan Cranswick <L.M.D.Cranswick...@dl.ac.uk>]

At 14:36 23/01/01 GMT, you wrote:
>I agree 100% with Armel about R-factors (and illustrate in particular
>about the effect on background subtraction in our PD web course at
>Birkbeck). I suspect the output of some programs are not very clear
>concerning R-factors (or even weights - since I suspect there are still
>many that use unit weights (i.e. non at all!)). I pointed out some
>of these problems recently at CDCC, but as I said during my talk,
>there will always be those who like to quote R-factors including
>background because the numbers are lower, and there will always be
>those who quote chi-squared values only becuase they wish to hide the
>fact that the data is not of the highest quality.
>Just the opinion of another powder crystalllographer,
>         Jeremy Karl cockcroft.
>

In the Crystals single crystal suite - part of the guided refinement
option is to encourage the user to apply an optimal weighting 
scheme (near the end of the refinement process).

   http://www.xtl.ox.ac.uk/crystals.html

Which weighting schemes are considered optimal for Rietveld refinement
and at what point in the refinement should they be switched on (unit
weighting(?) turned off)?

(There have been a recent paper or two on this but the user tends to
use whatever is available in their Rietveld program of choice?)

Brian Toby could correct my flawed memory here: Ted Prince was mentioning
(in the aftermath of a CCP14 talk) about "Robust Resistent Weighting 
Schemes" that could be applied throughout the refinement process
(but in another context - single crystal refinement).   How relevant
and used are these by Rietveld software?

---

Also: for Le Bail fitting applied to Unit Cell refinement (where you 
may want to try and weight each HKL of significant intensity in a 
roughly  equal fashion - small to high intensity peaks would be
considered significant), what would be considered a good weighting
scheme to use here?  Log or Root weighting of the counts?

Lachlan.

Lachlan M. D. Cranswick

Collaborative Computational Project No 14 (CCP14)
    for Single Crystal and Powder Diffraction
Daresbury Laboratory, Warrington, WA4 4AD U.K
Tel: +44-1925-603703  Fax: +44-1925-603124
E-mail: l.cranswick...@dl.ac.uk  Ext: 3703  Room C14
                           http://www.ccp14.ac.uk

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