On 7/23/13 9:01 AM, Edward A. Berry wrote:
>
> 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.
>
Isn't the reported Mean(I/sigI) in the reported Aimless table for the
merged/averaged reflections (because that is what we are discussing
about)? As an HKL2000 and XDS user (but not Scala/Aimless), I have
definitely seen adding more images (batches) improve the reported
I/sigma stats. I actually recently did a little educational
"experiment", where I kept on shooting, and scaling in XDS with more and
more batches, to determine at which point radiation damage was defeating
redundancy (in terms of I/sigI and CC1/2). Adding more batches (until a
certain point) definitely increased the XSCALE-reported I/sigma stats.
> 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."
>
Could it be that the reported <I/sigI> an average of the batch-wise
I/sigI's? I would love to hear that confirmed (or denied) by the authors
of Aimless.
Also, why would we be using the batch-wise I/sigma's for determining
resolution cutoffs?
> But I may be misunderstanding the point of that sentence.
Engin
|