Drift
Cohort drift after festival seasons
Journal · Switch Routehub · reading time about twelve minutes
Volume can look fine while a cohort quietly changes who it is. Festival seasons in Thailand — Songkran, Chinese New Year retail, late-year gifting, 11.11 — are the usual shock. Influencer weeks and LINE blasts are the cousin shock. If your drift alert only watches headcount, it will sleep through both.
Composition, not just size
In module 04 of Cohort Signal Architecture we ask for an alert on mix: category share, new-versus-returning ratio, unknown-id share, and the third-axis distribution. A “loyal” bucket that doubles in size because unknown cookies flooded a campaign is not a success. It is a naming crisis.
One residency team watched their flagship cohort stay within 4% of expected volume after 11.11 while unknown-id share jumped from 18% to 41%. Journeys kept sending. Complaints rose. The dashboard, which was volume-based, stayed green. The drift check that later shipped would have fired on unknown-id share alone.
Known shocks deserve a calendar
Do not ask a statistical process control chart to discover Songkran. Put known shocks on a calendar and change thresholds in those windows, then change them back. Treating festival volume as a new baseline is how RF inflation becomes next quarter’s target.
Monsoon delivery lags belong on the same calendar if you sell physical goods. Recency will look worse because parcels move slowly, not because affection died. A recency-only alert will mis-fire; a composition alert that includes delivery-exception rates will not.
Sunset clauses
Every named cohort in the Atlas carries a sunset: a date or a drift condition that kills the name. Without it, organisations accumulate labels the way they accumulate dashboards. After a festival, some names should die. That is not failure. That is maintenance.
If you cannot staff a weekly drift review, do not invent seven micro-segments before Songkran. You will not notice which ones dissolved. Start with fewer names, calendar-aware RF, and one composition alert. The flagship room is built around that discipline; the journal cannot sit beside your extract and argue with you.