>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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