linkage-by-era

Date: 2026-09-11. Status: superseded.

Decision. Linking events to SIMD by era follows the guidance's table 4, each event taking the edition recommended for its year and the record valid on its date. Events before 1996 get status no_edition with every SIMD column null, including the matched key. The Python and SQL implementations are tested to agree row for row on a synthetic cohort.

Reason. One rule, two implementations that cannot drift. Withholding the key on no_edition keeps the output columns consistent: no edition, no answer.

Evidence. tests/test_lookup.py ByEraPythonAndSqlAgree; docs/sql/link_by_era.sql; docs/LINKAGE_BY_ERA.md.