Re: [ccp4bb]: A simple question of resolution

["Manfred S. Weiss" <msweiss@EMBL-Hamburg.DE>]

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Dear Ed,

there is indeed a difference between a 'weak' reflection and a 
'non-existent' or rather 'non-observed' reflection. Check out the
old paper by Hirshfeld and Rabinovich (1973). Acta Cryst. A29, 517-25.

In a resolution shell where you have a average I/sigma(I) of let's
say 2.0 you have about 50% of all reflections with I/sigma > 2, about 
20% with I/sigma > 3 and so on, so it is reasonable to assume 
that the information that a reflection in this shell is weak or
very weak is actually meaningful. Leaving it out from whatever
you do with your data set will at least lead to wrong B-factors.

A shell which contains only weak reflection is probably beyond the
diffraction limit.

My feeling is that the 'optical resolution' is probably the parameter
which should be quoted, because it is least effected by what you 
actually define as the diffraction limit of your crystal.

Cheers, Manfred.


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On Thu, 25 Apr 2002, Edward A. Berry wrote:

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> 
> Phil Evans wrote:
> > 
> > Why use a cut off in refinement? This is unecessary and a Bad Thing to do
> > 
> 
> If one does not use a cut-off on F/SigF, why use a resolution cutoff?
> There is nothing intrinsically objectionable about High Resolution Data,
> the only reason I can see for excluding them is if they are weak (nonexistent).
> 
> Is it better to use a resolution cutoff which excludes a small number of 
> 5-10 sigma reflections in the last shell while keeping 0.5 sigma reflections
> at lower resolution, or a sigma cutoff which rejects weak reflections
> wherever they occur?
> 
> (this just to elicit more discussion- I am a firm believer in a zero F/sigF
> cutoff AND using resolution as far as truncate will allow). How to
> report the resolution without misleading is then a problem. I would
> favor Bart Haze's/Gerard Kleywegt's "effective resolution" based on the 
> number of reflections above some cutoff.
> 
> Ed
>