The
retention_start_date field was added in 2019, retroactively populated for prior years using a different method than the native calculation. This creates a subtle incomparability: 2019-onward retention is calculated one way, while earlier years were backfilled using a different approach. Should we recalculate the entire series using the current method?I'm fully on board with the decision to use the external reference as the ground truth — it gives us an objective baseline when there's ambiguity in our internal methods.