*** For details on how to be removed from this list visit the ***
*** CCP4 home page http://www.ccp4.ac.uk ***
Edward A. Berry wrote:
*** For details on how to be removed from this list visit the ***
*** CCP4 home page http://www.ccp4.ac.uk ***
Carlos Frazao wrote:
I understand that in earlier days, when data were much-much harder to
collect, crystallographers were very happy with complete (although
unredundant) data set. Then, Rmerge(or Rsym) were perfectly
adequated in pratical terms, as data collection expriments were
designed essentially to assure that at the end of the week data were
complete - it was not pratical to collect redundant data. Today, with
ccd's and synchrotron radiation, we quite often try to obtain data as
redundant as possible, in particular to detect anomalous signal, and
Rmerge statistics are simply not applicable anymore.
I just want to clarify that Rmerge or R-sym are _not_ appropriate at low
redundancy- they are appropriate at high redundancy (where they approach
the value of Rmeas) and at low redundancy they underestimate the
deviation
by a factor of up to sqrt(2)=1.41.
I apologise for not having been accurate in statement. But that was not
the issue I was trying to raise, but rather that in those times, due to
practical reasons, usually there were no highly redundant data in
macromolecular crystallography, and therefore the available statiscs
indicators didn't concern too much with the redundancy issue.
What about the physical meaning of Rmeas=Rrim ? If understood correctly
the Weiss papers, it indicates the "averaged precision of individual
measurements". How does this correlate with the paramter we should (I
believe) use instead: the "precision of the final averaged data"?
Wouldn't these two statistics increasingly differ with redundancy? Why
do we keep quoting statistics of the "individual measurements" when we
use instead a final averaged data set for structure determination and
refinement?
Carlos
--
**************************************
Dr. Carlos Frazao
Crystallography Department
ITQB-UNL, Av Republica, Apartado 127
2781-901 Oeiras, Portugal
Phone: (351)-214469666
FAX: (351)-214433644
e-mail: [EMAIL PROTECTED]
www.itqb.unl.pt
