At YC our motto is make something people want. But how do you actually know if you’ve accomplished that in the early days? One of the best ways to measure successful growth is a concept called cohort retention, which tracks the fraction of new users that come back time and time again to use your product. In this episode of Startup School, YC Group Partner David Lieb explains how to define cohorts, track active users and determine the appropriate time frame for measuring successful retention rates.