Engin Özkan wrote:
On 7/22/13 11:20 PM, Ethan Merritt wrote:
Which brings up the point that something seems to have
gone wrong in one of your processing runs.
Both runs claim mean (I/sigI) in the outer shell is 2.0,
but in one case this is for the 2.4A shell and in the other case
it's for the 2.2A shell. That is unlikely to be correct.
I/sigI should not depend on the Laue group.
Why is this surprising? This is what I would expect. By going to
orthorhombic, she is getting higher redundancy (two-fold more), and
hence higher I/sigI. She will also have fewer molecules per asu, and
less of NCS to help her with refinement if she sticks to orthorhombic.
Here we need some clarification on which I/s(I) is meant - sigma(I) for the
individual measurements, or sigma(I) from error propagation to the final
<I> which as Engin notes will be lower.
I thought it was the former that is to be used for selecting the cutoff, and
this is somewhat confirmed by the the recent Aimless paper (ActaD 69 1204-1214
"How good are my data and what is the resolution?" Philip R. Evans and Garib N.
Murshudov):
"The `maximum resolution' is estimated from the point at which <I/[sigma](I)> falls below 1.0 for each batch: note
that this <I/[sigma](I)> is without averaging multiple measurements (which would not generally occur on the same image),
so will be smaller than the <I/[sigma]> after averaging."
But I may be misunderstanding the point of that sentence.
Also note sigma(I) depends on your error model (at least for scalepack addicts)
But, other than that, yes, twin R factors should come with big warning
signs on them. Intensity moments, for example,
<I**2>/<I>**2 = 2.032
are showing no signs of twinning (and there is no translational NCS to
confuse the intensity stats), so the data should not have been treated
as twinned in the first place. I agree with Ethan that at first sight,
it appears orthorhombic.
Engin
