Day-N and Rolling Retention
The difference between classic, rolling, and bounded retention definitions.
Why Retention Has Multiple Definitions
An interviewer will rarely just say "compute retention". The sharp follow-up is: which retention? The same data yields very different numbers depending on the definition.
The three you must know: Day-N (classic) retention, rolling (unbounded) retention, and bounded-window retention. Knowing the difference, and asking which one the business wants, is itself the skill being tested.
Day-N (Classic) Retention
Day-N retention asks: was the user active on exactly day N after their first action? Day-1, Day-7, and Day-30 are the canonical mobile-app metrics.
The key word is exactly. A user active on day 6 and day 8 but not day 7 is not Day-7 retained under the classic definition. This precision is what makes the count strict and the curve jagged.
All lessons in this course
- Defining a Cohort by First Action
- Building a Retention Matrix
- Day-N and Rolling Retention
- Churn and Resurrection Queries