>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
