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When discussing this issue, perhaps we should not lose sight of the fact that the statistics behind Rp.i.m. assume 'independent observations'. Surely doing more than one rotation about the same axis is likely to repeat the same systematic errors? George Prof. George M. Sheldrick FRS Dept. Structural Chemistry, University of Goettingen, Tammannstr. 4, D37077 Goettingen, Germany Tel. +49-551-39-3021 or -3068 Fax. +49-551-39-22582 On Tue, 9 Dec 2008, Frank von Delft wrote: > Hi Manfred > > > > thanks a lot for your comments, since they raise some interesting > > points. > > > > R_pim should give the precision of the averaged measurement, > > hence the name. It will decrease with increasing data redundancy, > > obviously. The decrease will be proportional to the square root > > of the redundancy if only statistical errors or counting errors > > are present. If other things happen, such as for instance > > radiation damage, then you are introducing systematic errors, > > which will lead to either R_pim decreasing less than it should, > > or R_pim even increasing. > > > > This raises an important issue. As more and more images keep > > being added to a data set, could one decide at some point, > > when to add any further images? > > This really is the point: in these days of fast data collection, I assume > that most people collect more frames than necessary for completeness. At > least, I always do. So the question is no longer "is this data good enough" > -- that you can test quickly enough with downstream programs. > Rather, it is, "how many of the frames that I have should I include", so that > you don't have to run the same combination of downstream programs for 20 > combinations of frames. > > Radiation damage is the key, innit. Sure, I can pat myself on the shoulder by > downweighting everything by 1/1-N -- so after 15 revolutions of tetragonal > crystal that'll give a brilliant Rpim, but the crystal will be a cinder and > the data presumably crap. > > But it's the intermediate zone (1-2x completeness) where I need help, but I > don't see how Rpim is discriminatory enough. > > phx. > >