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Re: [ccp4bb]: Enforcing a sigma(f) cutoff



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>  what is a statistically significant difference in the R (or R-free) 
> statistic?  If the R and related factors can be influenced by the 
> solvent model, and in fact differ substantially when calculated by 
> different programs, should a difference of 2% in the R or R-free be 
> dismissed as insignificant?

For me, it is not uncommon to see an Rscale for presumably identical 
crystals around 2% to 3%, even for crystal data collected and processed 
presumably under identical conditions.  So if this in any way reflects 
the inherent error, I would be loath to call a 2% difference 
meaningful, but if it is random error, it should be just as likely to 
come out 2% lower as 2% higher.

Unless you have managed to keep the RMSD of bond lengths, angles, etc, 
fixed to the initial values of the model, it is hard to compare 
results.  I would want to do at least one cycle of refinement before 
deciding that the differences were due to anything more significant 
than different weighting schemes, bulk temperature and scaling factors 
for (potentially different) solvent masks, etc.