Why your firm should not aim at achieving 99% service level for all products (most likely)
Alejandro Serrano – . Mar 2012 | Spain
Last week I attended a nice talk given by the head of logistics of an e-commerce retailer. His firm holds roughly 10,000 SKUs and serves daily demand to end users from a central distribution center. A characteristic of the industry in which this firm operates is that orders should be satisfied in less than 24 hours. Since suppliers’ lead-times are on the weeks or even months (some are based in Asia), the firm is forced to hold large amounts of inventory to cope with uncertain demand.
The firm has a customer service level goal as high as 99% for all products. This figure, 99%, may be judged as appropriate by some people, but rises at least two questions: 1) why 99% and not, for instance, 90% or 99.9%, and 2) why 99% for all 10,000 products.
The answer to the first question might have to do with the fact that achieving 100% service level is virtually impossible. Therefore (put yourself in the CEO’s shoes,) if you want to provide your customers with an excellent service level, a feasible, close-enough-to-100%, easy-to-remember, and popular figure is 99%. And why not, you want to keep that figure high for all 10,000 SKUs in your warehouse.
A key point that is missed here is the fact that service level that maximizes expected profit should at least depend on the price and the cost of each particular item. In fact, inventory theory (or common sense) says that one should increase service level until the marginal benefit of adding an additional unit be exactly as high as the marginal cost of adding that additional unit. Note why this makes sense: given the cost of an specific item, if its market price goes up, the firm should increase the service level for that item. Why? a higher price implies a higher unit margin, and you want to capture that additional margin with higher probability, thus inventory should go up. Likewise, if given a price, the cost of an item increases, service level should be reduced to avoid a higher probability of holding too much inventory (as measured in moneys,) which is mainly driven by obsolescence, insurance, spoilage, and financial costs.
There are other factors that have an impact on service level above and beyond price and cost, such as the so-called salvage cost or goodwill cost. But as a conclusion, and without entering into details on how to compute optimal service levels, it should be apparent that 1) There is an optimal service level that depends on the margin of the product, which may be below or above 99%; and 2) service level should be computed for each SKU, or type of SKUs, thus defining it for all items in a firm does not make by and large much economic sense.
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