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[ccp4bb]: Ranom in Scala
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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?
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