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Yes of course; but the photons don't care what axis is being rotated about, they kill just as rapidly. phx. George M. Sheldrick wrote: > 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. >> >> >>