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.
