Measurement
Cohort
A cohort is a group of customers who share a starting characteristic — most often the period in which they made their first purchase — and are then tracked together over time.
Cohorting is what separates a change in the business from a change in the mix. A blended metric moves when new customers behave differently, when old customers behave differently, or when their proportions shift, and it cannot tell you which.
Cut cohorts by acquisition channel as well as date. Customers acquired on a heavy first-order discount retain differently, and blending them with organic ones makes both curves unreadable.
The classic misdiagnosis it prevents: overall retention looks flat, so the product team concludes nothing changed. Cohorting reveals that each new cohort is in fact retaining better while the mix has shifted toward a cheaper acquisition channel that retains worse — two large opposing movements averaging to a straight line. Acting on the flat number means fixing a product that is improving and scaling a channel that is not.