Hi Frank, of course R_pim is just one number and it may not be discriminatory enough to decide when to stop including images. But I can say from my own experience that R_pim will not drop forever. I have seen data sets with R_pim values of 0.5% to 2.0 A or better resolution, but never R_pim values significantly smaller than that (even with the redundancy approaching 100). You may try this out yourself. I bet you that R_pim will eventually go up again after a few revolutions when radiation damage kicks in. The real question in my opinion is, when is the deviation from the 1/(N-1) drop such, that you would want to stop including more images.
Cheers, Manfred. ******************************************************************** * * * Dr. Manfred S. Weiss * * * * Team Leader * * * * EMBL Hamburg Outstation Fon: +49-40-89902-170 * * c/o DESY, Notkestr. 85 Fax: +49-40-89902-149 * * D-22603 Hamburg Email: [EMAIL PROTECTED] * * GERMANY Web: www.embl-hamburg.de/~msweiss/ * * * ******************************************************************** 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. >
