How do you define retention, and why do cohort definitions matter?
PICTURE THIS: HOW TO EXPLAIN IT
Simple meaning
Retention is the share of a starting cohort that is still active after a stated window, such as day-7 return.
WHY — Business Metrics instead of guessing?
Why interviewers care about Business Metrics:
question about Business Metrics.
trade-offs, and what you would actually do on a Data Science project - not buzzwords.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1Retention is the share
of a starting cohort that is still active after a stated window, such as day-7 return.
- 2If the cohort mixes
new and resurrected users, or activity is defined inconsistently, the curve is not comparable.
- 3Plot retention by acquisition
week to separate product change from mix change.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
Retention is the share of a starting cohort that is still active after a stated window, such as day-7 return. If the cohort mixes new and resurrected users, or activity is defined inconsistently, the curve is not comparable.