On Sat, 2011-10-15 at 11:48 +0300, Nicholas M Glykos wrote:
> > > For structures with a small number of reflections, the
> statistical
> > > noise in the 5% sets can be very significant indeed. We have seen
> > > differences between Rfree values obtained from different sets
> reaching
> > > up to 4%.
This produces a curious paradox.
One possible reason for the variation in Rfree when choosing a different
test sets is that by pure chance reflections with more/less noise can be
selected. Which automatically means that the working set contains
reflections with less/more noise and therefore the model (presumably)
gets better/worse. So, selecting a test set that results in lower Rfree
leads to the model which is likely worse?
In fact, an obvious way to improve the Rfree through choice of a better
test set is by biasing it towards stronger reflections in each
resolution shell.
Selecting a test set that minimizes Rfree is so wrong on so many levels.
Unless, of course, the only thing I know about Rfree is that it is the
magic number that I need to make small by all means necessary.
Cheers,
Ed.
--
Oh, suddenly throwing a giraffe into a volcano to make water is crazy?
Julian, King of Lemurs
