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:[email protected]] On Behalf Of > Bernhard Rupp > Sent: Wednesday, June 26, 2013 13:31 > To: [email protected] > 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
