Dear Rain,
I will let XDS expert users provide the definitive response
to this because I'm not certain what exactly a "negative peak profile
correlation" indicates. However, I would be very cautious about deleting
reflections simply to improve the reported statistics. Taken to extremes, this
could lead to a suggestion to reject reflections which have a very poor
agreement with a model Fcalc to improve the Rfactors for refinement !
There needs to be a good physical reason for rejecting measurements. All the
software packages make an attempt to remove outliers, but judging what exactly
constitutes an outlier is far from trivial (one case that is straightforward is
when the backstop shadow has not been masked correctly during integration and
one measurement is OK while its symmetry mate is behind the shadow, so there is
a good physical explanation, but in many cases things are not so obvious).
What one needs to be very careful about is introducing bias. If there are, for
example, two symmetry related reflections, where because of genuine noise one
intensity is measured to be weaker than the other and you reject that weaker
observation, then you are biasing the intensity for that reflection towards the
larger value.
Simply reducing Rmeas and increasing I/sigmaI is absolutely not a guarantee
that the resulting dataset is actually better.
Andrew
On 31 Jul 2013, at 03:00, <Rain Field> <[email protected]> wrote:
> Hi All,
> Inspired by the "micro diffraction assembly" methods (see
> http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12357.html), I
> checked one XDS_ASCII.HKL file and found many reflections has negative peak
> profile correlation. After deleted them and rerun XSCALE, I/sigma is higher
> and Rmeas is lower in the same high resolution shell than without deletion.
> I am wondering why it's not a common practice to delete those reflections?
> Thanks!
