Compute SigAno = <|I(+)-I(-)|>/<sigma(I(+)-I(-))> per report shell and overall
from the full-multiplicity I(+)/I(-) Bijvoet split, store it on
MergeStatisticsShell, and surface it in the console merge-statistics table (new
SigAno column) and the mmCIF via the standard wwPDB PDBx items
_reflns.pdbx_absDiff_over_sigma_anomalous (overall) and
_reflns_shell.pdbx_absDiff_over_sigma_anomalous (per shell) - the dictionary's
home for this quantity, so no jfjoch_ local item is needed (unlike ISa).
SigAno approaches ~0.8 for pure noise and rises above 1 with real anomalous
signal. A half-set anomalous CC is deliberately not used: its two half estimates
are complementary partitions of one observation pool (dI0 + dI1 = 2*dI_full), so
differencing the two Bijvoet hands cancels the large common intensity that keeps
CC1/2 non-negative and, once the anomalous SNR per half drops below 1, drives the
correlation towards -1 rather than 0 - misrepresenting a weak-but-real signal.
Emitted only when an anomalous split was made (has_anom guard), so a non-anomalous
merge keeps its stats block and shell-loop columns byte-for-byte. Export-neutral:
the I(+)/I(-)/IMEAN accumulation is unchanged. On low-energy S-SAD test data SigAno
tracks XDS (13 keV null ~0.89, 6 keV ~1.44, 5 keV ~1.5-1.7) and never goes
spuriously negative.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
leonarski_fandClaude Opus 4.8
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