While all inventories are subject to obsolescence, there are some products that are considered highly peri able in that they are only viable for a very short time period. After that period has elapsed. their value decreases significantly, and the reduced value is often only their salvage or scrap value. Examples of such products include newspapers and Christmas trees. In each case. the value of the product drops dramatically from one-time period to the next. (In the case of newspapers, the time period is a day; in the case of Christmas trees, it is the month or so before Christmas.) The determination of the proper or "optimum" inventory level to establish for this group of products is often referred to as the single period inventory model.