Re: [ccp4bb]: R-sym I/sigI etc...

[Bart Hazes <bhazes@ualberta.ca>]

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On Thu, 1 Aug 2002, Roberto Steiner wrote:

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> Maybe naive but if you have refined your structure and in your outermost shell
> you have R factors lower that 59% I would say there is a non random
> contribution in that shell and therefore no need to cut.
>
> Roberto

You'll have to look at your Free Rfactor, as the traditional Rfactor will be
lower than random just due to model bias. The Rfactor, on amplitudes, for a
random match is closer to 54% I believe.

If you don't have a lot of Rfree reflections at the highest resolution range
so you're not sure if you can trust the statistics to judge whether or not
there is real information in the data, you could be a bit more
conservative/realistic in selecting your max resolution at first and then
compare Fcalc and Fobs of the reflections beyond that resolution limit to see
if you should include them in subsequent refinements.

Wrt Jim's earlier comments; I think there have now been a number of cases that
clearly show that we should be including weaker data than was previously
considered "best practise". The best being some ARP/wARP runs that only worked
when the weak data was included. Maybe they should ensure that the difference
was not just due to the finer FFT grid you get with the higher resolutions,
but my personal experiences also indicate that Rsym & I/SigI are not the
definitive criteria on which to base your resolution cut-off for refinement.
Some kind of standard for reporting resolution in papers would still be
helpful but given that we are still talking about this after 50 years of
crystallography makes it is clear that this is not clear.

Bart

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