Assessing purchase timing models: Whether or not is preferable to when

Two types of purchase timing models-those which model purchase incidence (whether or not) and those which model interpurchase time (when)-are examined. The authors show that modelling purchase incidence is almost always preferable to modelling interpurchase time. They also derive upper bound R²s for purchase incidence models. These R²s serve as useful benchmarks for assessing the goodness-of-fit of empirical choice models.