Re: [ccp4bb]: A simple question of resolution

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

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Bernard Santarsiero writes:
 > I process the data to at least I/sig of 1.0, and then look at the
 > completeness at the highest resolution shell.  If you use some kind of
 > strategy program, you should be able to get good completeness.  The main
 > refinement programs, like CNS, use a 1 or 2 sigma cutoff for refinement.
 > So if I process the data to 1sig, then I refine to 1sig. 
 > 

Why use a cut off in refinement? This isunecessary and a Bad Thing to do


 > I also don't monkey around with the weights when processing the data.  
 > Trying to make the chi**2's all 1 just assumes that random errors are
 > the dominating errors.  I think statistical errors are at least as large
 > as the random errors, and have an unknown distribution.  It's probably not
 > Gaussian, so you can't make the chi**2's 1.  We've collected small
 > molecule data on in-house detectors and at the beamline, and have some
 > idea of what the true weights (and weight contributors) are.

Not monkeying around assumes the initial estimate is correct & it isn't

Jim Pflugrath writes:
 > We define in our structural biology contracts that the maximum resolution
 > is where the average I/sigmaI for the high resolution shell is 2 for all
 > possible reflections in the shell.  Please note the use of the word
 > average.  
 > 
 > Furthermore, the average I/sigI is calculated before averaging
 > redundancies.  That is, the calculation is made on the input reflections
 > and not on the averaged unique reflections that are output and used in
 > subsequent calculations.  The average I/sigI for the output averaged
 > reflections will be a function of the input unaveraged I/sigI and the
 > redundancy (i.e. multiplicity) of the input reflns.
 > 

This is a very fierce cutoff. If you are going to cutoff on I/sigI
_before_ averaging, how are you gaining by collecting high redundancy?


Phil