Survivorship Bias
A current-status filter can hide former members. Recovering a historical population depends on retained membership history and explicit missing-data checks.
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Cohort-Based LTV
Build cohort value from period totals, use a fixed customer denominator, and make the forecast horizon and unobserved tail explicit.
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Retention
Retention depends on the starting population, return event and time window. Revenue can grow even while the number of retained customers falls.