Birthday Retention
AdvancedBirthday retention is another name for N-day retention: a user counts as retained on day N only if they were active on exactly that day.
The name comes from treating day N as the cohort's birthday: only attendance on that exact date counts, and activity on the day before or after adds nothing. It is the strictest retention definition and therefore produces the lowest curve. It is also the noisiest, because day-of-week effects land on particular day numbers, so a product used on working days shows artificial bumps at day 7 and day 14 and a curve that rises and falls. The main error is comparing this curve with a rolling or unbounded one, since the gap between them is a difference of definition, not a decline in retention.
How it is calculated
Birthday retention on day N = cohort users active on exactly day N ÷ cohort size
Day N is counted from each user's own day zero, not from a shared calendar date. The unit is a percentage and every day is evaluated independently, so unlike unbounded retention the curve can go up as well as down. On small cohorts a single day's value swings widely and reading one day in isolation is unreliable.