Why it matters
Blended metrics smear the past over the present: retention can be collapsing for new customers while the overall average looks fine, propped up by loyal old-timers. Cohorts reveal whether things are getting better: the question every strategy change begs.
How it works
Each row is a joining period, each column an age; cells show survival, spend or activity at that age. Reading down a column compares generations at the same life stage: the clean way to see if the new onboarding, pricing or positioning changed anything.
What to do about it
After any significant change, judge it by cohorts: do customers acquired after behave differently at the same age? It's the closest thing marketing has to a before-and-after experiment.
