Many inventory systems in practice are confronted with lost
sales if demand cannot be delivered instantaneously from inventory on hand. In a worldwide survey on out-of-stocks in the fast
moving consumer goods retail sector for example, Gruen et al.
(2002) show that only 15% of all customers delay the purchase if
they are faced with a stock-out for their preferred product in a
particular store. The other 85% of the customers decide to buy
another product (substitution), buy the product in another store
or not buy the product at all. In all these cases demand for the
preferred product–store combination is lost.