With inventory, you're never deciding whether to drop a product; rather, you're deciding when to do so. Think about the last time you went grocery shopping. Did you notice that most food items had sell by (expiration) dates stamped on their packaging? If a product's sell by date has passed, the product shouldn't be purchased. Instead, it's eventually replaced on the store's shelves by a fresher product.
Most products (food or otherwise) have expiration dates, or at least life spans, because at some point, for whatever the reason, products are no longer valid. In marketing terms, a product's life span is referred to as its life cycle (see Figure 3-1).
Typically, a product's life cycle is defined in four stages: