What does a First Time Retention report mean, and how does Mixpanel know that this is the first instance of the user firing the Event?

Mixpanel’s First Time Retention report allows you to select one Event as a cohortizing Event and another Event on which to calculate user retention. This report is useful for looking at users who performed a specific Event once (such as sign up) and how often they come back to do more Events.

The First Time Retention report retrieves the first instance of the cohortizing Event in the defined time range, but it doesn’t take into account whether it’s the first time all-time a user has done a certain action. For example, if you select “Add payment method” as your cohortizing event, the cohorts will include all users who did the Event “Add payment method” during the selected time frame, not the first-time in that user’s lifecycle.

Read more about Retention reports and how they’re calculated.