Choosing Metrics for a Product

A language learning app has built a review feature. After each lesson, the app schedules a short review of the words the learner got wrong, and it sends a reminder when the review is due. The team will show the feature to half of its new learners for 14 days. When the test ends, which number should decide whether the feature ships?

The obvious candidate is the number of reviews that learners complete. This lesson shows why that number cannot decide anything