Hi Bernhard,
The formula from Tickly applies to the weighted/generalized/Hamilton free
R-factor. From k-fold cross validation tests we observed that the 'regular'
R-free has a standard deviation of R-free*(Nref )^-1/2
Cheers,
Robbie
> -----Original Message-----
> From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of
> Bernhard Rupp
> Sent: Wednesday, June 26, 2013 13:31
> To: [log in to unmask]
> Subject: Re: [ccp4bb] Rfree is 20%,why still green and red density?
>
> >you may have only a few hundred and thus not get a reliable Rfree value.
>
> The estimate for the error in R free as a function of the number of
reflections
> is as follows:
>
> Brunger initially estimated^35 that the uncertainty in R-free is
proportional to
> (Nref )^-1/2, which is reasonable to assume because this is how
uncertainties
> vary with sample size. Tickle et al. finally showed^38 that the relative
> uncertainty in Rfree is exactly equal to (2Nref )^-1/2 confirming
Brunger's
> initial estimate, with constant of proportionality as 2^-1/2.
>
> Following this proportionality, ~1000 reflections are sufficient to obtain
a
> better than 1% precision for an overall R-free in the 20-30% range, i.e.
'a few
> hundred' is still not too bad.
>
> Best, BR
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