A rotation run makes the space-group decision twice: once on a merge of only
the well-measured observations (--search-min-zeta) and once on all of them, and
the rule was to keep whichever search found more symmetry. Its justification
was that the filtered arm can only ever LOSE an operator - discarding 40-80% of
the observations starves the operator correlations - and never invent one.
That premise was checked at one set of geometries, and there it holds exactly:
over 1232 stored battery runs (38 geometries x 32 code variants) the two arms
disagree 59 times and the all-observation arm is the higher one every single
time. Away from those geometries it fails. Over 68 runs whose first pass was
given a displaced beam centre, the filtered arm confirms an operator the full
merge refuses 14 times, and the two arms never once both confirm the promotion.
On one rhombohedral crystal the all-observation merge refuses a 3 -> 32
promotion at the correct beam centre, on the same twin-law statistic the search
uses everywhere (1.78 against a bound of 1.70), and the filtered merge - which
had thrown two thirds of the observations away - overrides it into the wrong
space group. The rule was validated on the only data that cannot test it.
So the filtered arm no longer promotes. Where the two disagree the answer is
the one the merge with all the observations supports, as the systematic
absences already were, and both are still reported. The filter keeps the job it
was added for - stopping a near-tangential measurement from making a real
operator look like a twin law - it simply cannot outvote the merge that has
every observation in it.
Costless on the 38-crystal battery, as predicted: the changed branch is never
taken there (41 searches: 39 agree, 2 with the all-observation arm higher, none
the other way), the space groups stay at 35/38 with the same three misses, and
the crystals the second search does rescue - where the all-observation merge is
the one finding the higher symmetry - are untouched. Injecting the post-refined
beam centre into the crystal above now yields the right space group, 27900
unique reflections at R_meas 12.6% against 14078 at 21.3% before.
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