This article is concerned with counting and identifying those customers who are still active. The issue is important in at least three settings: monitoring the size and growth rate of a firm's ongoing customer base, evaluating a new product's success based on the pattern of trial and repeat...

Two types of purchase timing models—those which model purchase incidence (whether or not) and those which model interpurchase time (when)—are examined. We show that modelling purchase incidence is almost always preferable to modelling interpurchase time. We also derive upper bound ²'s for...