Cohort
A group of subscribers who share a common signup window or trigger — the unit of retention measurement.
A cohort, in email analytics, is a group of subscribers who share a common signup window or triggering event — January 2024 signups, all subscribers from a specific campaign, all users who hit a paywall in week 32, etc. Cohort analysis tracks how the group's engagement decays over time, which is impossible to see in pooled metrics.
Cohort analysis reveals truths that aggregate metrics hide. A list with declining overall open rates might actually have rock-solid retention within each cohort — the decline is just dilution from large recent cohorts that haven't yet engaged. Conversely, a list with stable aggregate metrics might be hiding rapid churn within each cohort, masked by new signup volume.
For mature email programs, cohort analysis is the way to measure long-term list health. Weekly or monthly cohort-engagement curves are more honest than overall metrics. Most ESPs surface cohort views; if yours doesn't, the analytics layer is worth building.