[ccp4bb]: Ranom in Scala

[Phil Evans <pre@mrc-lmb.cam.ac.uk>]

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Dr Nicholas Keep writes:
 > 
 > I am processing nearly 180 degrees of data from a crystal which I
 > believe to be P422 (hence I have very high redundancy).  I am perplexed
 > by the behaviour of Ranom in Scala.  Processing my data gives Rfac in
 > the outer bin of .375 Rfull of 0.358 and Ranom of 0.10.  This made me
 > wonder if the crystal was not showing the full symmetry at the high
 > resolution as the anomalous data is so much 'better' so I reprocessed in
 > P1 (still fairly complete data).  This only dropped the Rfac in the
 > outer bin to 0.227 rfull to 0.246 but increased Ranom to 0.303.  These
 > trends generally hold in the lower resolution bins and for the peak and
 > remote wavelengths.
 > 
 > Is the explaination of this that the Ranom is based on the average I+
 > and I- for the merged equivalents rather than the matched Bijvoet pairs
 > for a particular index before merging?
 > 

Nick

	I think you're right: Ranom is based on <I+> and <I-> unlike
Rfac - I'm not sure this is a particularly useful statistic (so why
did I put it in, I wonder?). But I think anyway the only way to test
for pseudo as opposed to crystallographic symmetry is to merge in
different pointgroups (use "reindex" to change symmetry & resort).
Rmeas may be a better criterion than Rfac

	Phil