The resolution cutoff, the shell table and the overall CC1/2 counted every
unique reflection equally. The merge itself is inverse-variance weighted, so
an observation from a frame the crystal barely diffracted on enters it at
1/G^2 of a good one - honestly, with its sigma - but a reflection measured
only on such frames is scaled-up noise that then counts as much as a
well-measured pair in every Pearson CC1/2 read off the merge. On a sweep
where half the frames are weak the curve collapses at every resolution: the
cut lands at 3.7 A on a P1 crystal whose good frames reach 1.5 A, and the
UNUSABLE verdict fires (CC1/2 0.40 beside I/sigma 10).
Each merged reflection now carries cc_weight: the precision its half-sets
would have had with every observation at the run's typical frame scale, over
the precision they have. G_ref = sum G^3 / sum G^2 over the usable
observations is the precision-weighted typical scale, which the dead frames
cannot drag down however many there are; the factor per observation is
max(1, (G_ref/G)^2), with G the frame's total scale (partial scale, flux and
the fulls' own G) taken before the correction surfaces and before collapsed
frames are dropped, so nothing intensity- or resolution-dependent enters it.
On a sweep without a weak stretch every weight is 1 and the CC1/2 is the
plain Pearson it was. The cutoff fit, the shell table and the overall CC1/2
(and so the UNUSABLE verdict and the report's shell checks) all read the same
weighted statistic.
The two extra per-group sums are accumulated on both merge paths, the host
loop and MergeAccumKernel, from one per-frame factor array; a host recompute
of the device sums agrees to 1e-15 relative on every merge after the
corrected corr is uploaded, and a host-merge run gives the same cut, space
group, CC1/2 and ISa on five sets.
Weighting by the half-set error variance alone (1/(v0+v1)) is not this: v
grows with the intensity, so it weights the weak end of the intensity
distribution and biases homogeneous data coarser.
Measured (written resolution, together with the weighted outlier median):
a P1 sweep with a long weak stretch 3.73 -> 1.40 A against a 1.63 A XDS
reference, UNUSABLE withdrawn (overall CC1/2 0.40 -> 0.97); a second 3.61 ->
3.00 A; one with most of the sweep out of beam 7.35 -> 5.20 A. Lysozyme,
thaumatin and two insulin sets unchanged to 0.01 A (one lysozyme sweep with a
weak wedge 1.13 -> 1.16 A, from the median), same space groups throughout.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
leonarski_fandClaude Opus 5
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