You may have a look inside the Rietveld FAQ and discussions
gathered at :
http://www.unige.ch/crystal/stxnews/riet/faq/rietfaqs.htm
by Lachlan Cranswick.
A starting point may be the "Low FWHM and Rp discussion". If
you have not access to the WEB, it is available by anonymous ftp
at aviion.univ-lemans.fr/pub/fluorlab/LOW_FWHM_AND_RP.TXT
as an ASCII file -28K-(consider the discussion is now one year old,
think to add your own conclusions).
Armel http://fluo.univ-lemans.fr:8001/