Re: [ccp4bb]: A simple question of resolution

["Edward A. Berry" <EABerry@LBL.gov>]

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"Manfred S. Weiss" wrote:
> 
> 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.

And if truncate refuses to process the data because <I> is negative
in a shell, we've gotten well into that region
(I agree with all of this. The distinction between "a zero F/sigF
cutoff" and "no cutoff" is semantic since F is always positive.) 

> 
> 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.
>

Yes, I realized after posting that if you use "effective resolution"
based on number of reflections above a reasonable(?) cutoff like 0.5 or
1 sigma, resolution shells with no useful data will by pure statistics 
contribute to increasing the resolution. If you use a cutoff of 2 sigma
or higher, the result will be overly "sobering" and not at all realistic,
since we would never use such a severe cutoff in refinement nowadays.

Ed

> 
> On Thu, 25 Apr 2002, Edward A. Berry wrote:
> 
> > ***  For details on how to be removed from this list visit the  ***
> > ***          CCP4 home page http://www.ccp4.ac.uk         ***
> >
> > 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
> >